Chapter 2 of 8

All 8 chapters
  1. 1Sorting living things, readunderstood
  2. 2Using and making keys, readunderstood
  3. 3Food webs, readunderstood
  4. 4Energy through a food chain, readunderstood
  5. 5Decomposers and recycling, readunderstood
  6. 6Competition and adaptation, readunderstood
  7. 7Counting living things, readunderstood
  8. 8People and ecosystems, readunderstood

S3.2.2 Chapter 2

Using and making keys

I found a small creature under a stone. How can I find out its name?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Use a branching key to name an unknown leaf or insect
  • Make a key with yes or no questions for four animals

Naming a living thing is hard when there are so many. A key helps. It is a set of yes or no questions. Each answer leads you along a branch, and each branch ends at a name.

This key is for four small animals only. Watch it grow, one row at a time.

Draw it first

A key for four small animals

  1. A branching key. The first question is Has wings? Yes leads left to bee. No leads right to the question Has 8 legs? Yes leads to spider. No leads to the question Has a shell? Yes leads to snail and no leads to worm. Labels: Has wings?, Has 8 legs?, Has a shell?, bee, spider, snail, worm, yes, no.Has wings?

    1 Start at the top with one question. Does the animal have wings?

  2. A branching key. The first question is Has wings? Yes leads left to bee. No leads right to the question Has 8 legs? Yes leads to spider. No leads to the question Has a shell? Yes leads to snail and no leads to worm. Labels: Has wings?, Has 8 legs?, Has a shell?, bee, spider, snail, worm, yes, no.yesbeenoHas 8 legs?Has wings?

    2 Yes goes left to bee. No goes right, to the next question.

  3. A branching key. The first question is Has wings? Yes leads left to bee. No leads right to the question Has 8 legs? Yes leads to spider. No leads to the question Has a shell? Yes leads to snail and no leads to worm. Labels: Has wings?, Has 8 legs?, Has a shell?, bee, spider, snail, worm, yes, no.yesbeeyesspidernoHas a shell?noHas 8 legs?Has wings?

    3 The next question is about legs. Yes goes to spider. No goes down.

  4. A branching key. The first question is Has wings? Yes leads left to bee. No leads right to the question Has 8 legs? Yes leads to spider. No leads to the question Has a shell? Yes leads to snail and no leads to worm. Labels: Has wings?, Has 8 legs?, Has a shell?, bee, spider, snail, worm, yes, no.yesbeeyesspideryessnailnowormnoHas a shell?noHas 8 legs?Has wings?

    4 The last question is about a shell. Yes is a snail. No is a worm.

Every animal in the key has one path to its name, and no other. To use the key, look at your animal and answer each question truthfully. Do not skip a question.

A table of clues for four animals. The bee has wings, 6 legs and no shell. The spider has no wings, 8 legs and no shell. The snail has no wings, 0 legs and a shell. The worm has no wings, 0 legs and no shell. Labels: animal, wings, legs, shell, bee, spider, snail, worm, yes, no, 6, 8, 0.animalwingslegsshellbeeyes6nospiderno8nosnailno0yeswormno0no
The clues the key uses. Each question asks about one column.

A key also works for plants. Look at a leaf. Is it shaped like a needle? Is it long and thin? Does it have many small leaflets? The heavy line shows the way to a fern.

A key for four leaves. The first question is Needle shaped? Yes leads to pine. No leads to Long and thin? Yes leads to grass. No leads to Many leaflets? Yes leads to fern and no leads to lotus. A heavy line marks the way to fern. Labels: Needle shaped?, Long and thin?, Many leaflets?, pine, grass, fern, lotus, yes, no.yespineyesgrassyesfernnolotusnoManyleaflets?noLong and thin?Needle shaped?
A leaf key. The heavy line is the path to fern.

Now make your own key. Choose questions with a clear answer that you can see. Wings, legs, fins, feathers and a shell are good clues. Colour and size are poor clues. They change from one animal to the next of the same kind. A key like this one works only for the things in it.

Two boxes of questions. Good questions: Has wings?, Has 8 legs?, Has a shell?. Poor questions: Is it nice?, Is it big?, Is it scary?. Labels: good, poor, Has wings?, Has 8 legs?, Has a shell?, Is it nice?, Is it big?, Is it scary?.goodHas wings?Has 8 legs?Has a shell?poorIs it nice?Is it big?Is it scary?
A good question has a clear answer that is the same for everyone.

See it three ways

With things

Put four animals in a row. Ask a question that sorts out one of them. The rest stay in a group.

As a picture

Two boxes. The box labelled yes to wings holds bee. The box labelled no to wings holds spider, snail and worm. One question has split four animals into two groups. Labels: yes to wings, no to wings, bee, spider, snail, worm.yes to wingsbeeno to wingsspidersnailworm

In words

Each question splits the animals into two groups. The next question splits one group again. You stop when every group holds one animal.

In symbols

Four animals need three questions: 4 − 1 = 3. A key for n animals has n − 1 questions.

Try it

Make a key for four animals

You need: a pencil, a sheet of paper, four animals you know, such as a hen, a fish, a dog and a snake.

  1. Write the four names. Find a question that is true of only one of them.
  2. Draw the question in a box. Draw a branch to the left for yes, with that name at the end.
  3. Draw a branch to the right for no. Write a new question that sorts out the next animal.
  4. Keep going until every animal has its own end. Ask a friend to name an animal with your key.

What you will see: Three questions are enough for four animals. If two animals end in one place, you need one more question. If a question has no clear answer, change it.

If you cannot try it, this is what happens.
A key made for four animals. The first question is Has feathers? Yes leads to hen. No leads to Has fins? Yes leads to fish. No leads to Has legs? Yes leads to dog and no leads to snake. Labels: Has feathers?, Has fins?, Has legs?, hen, fish, dog, snake, yes, no.yeshenyesfishyesdognosnakenoHas legs?noHas fins?Has feathers?

A wrong idea many people have

Questions about colour or size make a good key.

Why it feels right: Colour and size are the first things you see.

The test that breaks it: Look at the ants from one nest. Some are big and some are small. No one answer about size is true of every ant.

What is true: Choose clues that stay the same, such as the number of legs, wings or a shell. A key is only as good as its questions.

Key words

key
a set of yes or no questions that leads you to the name of a living thing.
branch
one path in a key, from one answer to the next question or to a name.

Back to your guess

The answer is A: Answer yes or no questions, one at a time, until only one name is left.

A key uses yes or no questions about clear clues. Each answer rules some names out, until one name is left. Colour and size are poor clues, because they vary in one kind of animal.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. An insect has wings. They are hard wing cases. This key is for these four insects only. Use it to name the insect.

    A key for four insects. The first question is Has wings? Yes leads to Hard wing cases? Yes leads to beetle. No leads to Big, patterned wings? Yes leads to moth and no leads to bee. No to Has wings? leads to flea. Labels: Has wings?, Hard wing cases?, Big, patterned wings?, beetle, moth, bee, flea, yes, no.yesbeetleyesmothnobeenoBig,patternedwings?yesHard wing cases?nofleaHas wings?
    Tap your answer.

    1. Not this one. You may have taken the wrong turn at the first question. This insect has wings, so the answer is yes and the path goes left. A flea is at the end of no. Go to S3.2.2. Try another.

    2. Yes, that is right. Yes to wings, then yes to hard wing cases. That path ends at beetle.

    3. Not this one. A moth has big, patterned wings but no hard cases. This insect has hard wing cases, so the answer to that question is yes. Go to S3.2.2. Try another.

    See the answer

    beetle

    Yes to wings, then yes to hard wing cases. That path ends at beetle.

    • If you chose A: You may have taken the wrong turn at the first question. This insect has wings, so the answer is yes and the path goes left. A flea is at the end of no. Go to S3.2.2.
    • If you chose C: A moth has big, patterned wings but no hard cases. This insect has hard wing cases, so the answer to that question is yes. Go to S3.2.2.

  2. A leaf is not a needle. It is not long and thin. It has many small leaflets. This key is for these four leaves only. Which plant is it?

    A key for four leaves. The first question is Needle shaped? Yes leads to pine. No leads to Long and thin? Yes leads to grass. No leads to Many leaflets? Yes leads to fern and no leads to lotus. Labels: Needle shaped?, Long and thin?, Many leaflets?, pine, grass, fern, lotus, yes, no.yespineyesgrassyesfernnolotusnoManyleaflets?noLong and thin?Needle shaped?
    Tap your answer.

    1. Not this one. Grass is the end of yes to long and thin. This leaf is not long and thin, so the path goes on to the next question. Go to S3.2.2. Try another.

    2. Yes, that is right. No to needle, no to long and thin, then yes to many leaflets. That path ends at fern.

    3. Not this one. Lotus is at the end of no to many leaflets. This leaf has many leaflets, so the answer is yes. Go to S3.2.2. Try another.

    See the answer

    fern

    No to needle, no to long and thin, then yes to many leaflets. That path ends at fern.

    • If you chose A: Grass is the end of yes to long and thin. This leaf is not long and thin, so the path goes on to the next question. Go to S3.2.2.
    • If you chose C: Lotus is at the end of no to many leaflets. This leaf has many leaflets, so the answer is yes. Go to S3.2.2.

  3. An animal has no wings and not 8 legs. It has a shell. Use the animal key. Which animal is it?

    A branching key. The first question is Has wings? Yes leads to bee. No leads to Has 8 legs? Yes leads to spider. No leads to Has a shell? Yes leads to snail and no leads to worm. Labels: Has wings?, Has 8 legs?, Has a shell?, bee, spider, snail, worm, yes, no.yesbeeyesspideryessnailnowormnoHas a shell?noHas 8 legs?Has wings?
    Tap your answer.

    1. Yes, that is right. No to wings, no to 8 legs, yes to a shell. That path ends at snail.

    2. Not this one. A spider is at the end of yes to 8 legs. This animal does not have 8 legs, so the path goes down to the next question. Go to S3.2.2. Try another.

    3. Not this one. A worm is at the end of no to a shell. This animal has a shell, so the answer there is yes. Go to S3.2.2. Try another.

    See the answer

    snail

    No to wings, no to 8 legs, yes to a shell. That path ends at snail.

    • If you chose B: A spider is at the end of yes to 8 legs. This animal does not have 8 legs, so the path goes down to the next question. Go to S3.2.2.
    • If you chose C: A worm is at the end of no to a shell. This animal has a shell, so the answer there is yes. Go to S3.2.2.

  4. Which is the best question to use in a key for animals?

    Tap your answer.

    1. Not this one. People do not agree about pretty. A good question has a clear answer that is the same for everyone. Go to S3.2.2. Try another.

    2. Not this one. Big depends on what you compare it with, and animals of one kind differ in size. Choose a clue that does not change. Go to S3.2.2. Try another.

    3. Yes, that is right. You can see wings, and an animal either has them or does not. The answer is yes or no for every animal.

    See the answer

    Does it have wings?

    You can see wings, and an animal either has them or does not. The answer is yes or no for every animal.

    • If you chose A: People do not agree about pretty. A good question has a clear answer that is the same for everyone. Go to S3.2.2.
    • If you chose B: Big depends on what you compare it with, and animals of one kind differ in size. Choose a clue that does not change. Go to S3.2.2.

  5. You make a key for 6 animals. Each end of the key holds one animal. How many questions does the key have?

    Tap your answer.

    1. Not this one. You may be giving one question to each animal. The last two animals share a question, because its two answers lead to one animal each. Go to S3.2.2. Try another.

    2. Yes, that is right. Each question adds one end. A key for 6 animals has 6 ends, so it has 6 − 1 = 5 questions.

    3. Not this one. A key never needs more questions than animals. Each question splits one group into two, and that adds one end. Go to S3.2.2. Try another.

    See the answer

    5 questions

    Each question adds one end. A key for 6 animals has 6 ends, so it has 6 − 1 = 5 questions.

    • If you chose A: You may be giving one question to each animal. The last two animals share a question, because its two answers lead to one animal each. Go to S3.2.2.
    • If you chose C: A key never needs more questions than animals. Each question splits one group into two, and that adds one end. Go to S3.2.2.

  6. From an earlier chapter

    Which of these animals is an invertebrate?

    Tap your answer.

    1. Not this one. A goat has a backbone, so it is a vertebrate. Go to S3.2.1. Try another.

    2. Not this one. A fish has a backbone, so it is a vertebrate. Go to S3.2.1. Try another.

    3. Yes, that is right. A snail has no backbone, so it is an invertebrate.

    See the answer

    A snail

    A snail has no backbone, so it is an invertebrate.

    • If you chose A: A goat has a backbone, so it is a vertebrate. Go to S3.2.1.
    • If you chose B: A fish has a backbone, so it is a vertebrate. Go to S3.2.1.

Keep it

Copy this into your exercise book.

A key is a set of yes or no questions. Each answer leads along a branch to a name. Choose clear clues such as legs, wings or a shell. A key for n animals has n − 1 questions.

Draw: Draw a key for a bee, a spider and a worm. Write each question in a box with yes on the left and no on the right.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (860 KB). It opens on any phone.