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# Who is it? Using a key

Year 7 Biology · Classification

<!-- Speaker note: Students use a dichotomous key to identify organisms, name the feature at each decision, and explain how a wrong choice leads to a wrong name (AC9S7U01, node 3, a hinge node). About 25 minutes (Engage and Explore 9 min · Explain 8 min · Evaluate 3 min · Typical adds 4 min, Wider about 2 min). -->

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## Today

**We are learning to** a dichotomous key identifies an organism by a series of yes-or-no questions about features you can observe. Following each choice carefully leads to the right name; one wrong choice leads to the wrong one.

- I can use a dichotomous key to identify at least five organisms.
- I can name the observable feature used at each step.
- I can explain why a wrong choice leads to a wrong name.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace keys with real specimens or a field guide outside. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge risk is procedural key-following: students reach a name without being able to say what each step checked, and then build broken keys in node 4. Keep asking "what feature did that step check?". Board notes: specimens are anonymous descriptions; names appear only when the key is followed correctly. A wrong turn names the first step where the student's path left the right one. Teacher: In your own words: the platypus path is the evidence. -->

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## Why it matters

> A field guide doesn't show you every animal in Australia and ask you to find the match. It asks you a question about the one in front of you, then another, then another.

<!-- Speaker note: Last lesson: kingdoms. This lesson: a tool for naming organisms one careful choice at a time. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**You have a dichotomous key and an animal you don't know. How do you think you use the key?**

- Look for the picture that matches
- Guess the name, then check it in the key
- Count its legs and look up the number
- Answer two-way questions about the animal's features, one at a time, until the key names it

<!-- Speaker note: What happens: Answer two-way questions about features you can observe, one at a time. Each answer sends you to the next question, until you reach a name. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: if anyone chose 'guess the name, then check', point out that the board hides names on purpose. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-7%2Fkeys%2Fstep-2&code=widget%3A+key%0Astep+A%3A+Does+it+have+a+backbone%3F+%7C+B+%7C+F%0Astep+B%3A+Does+it+have+feathers%3F+%7C+Kookaburra+%7C+C%0Astep+C%3A+Does+it+have+fur%3F+%7C+D+%7C+E%0Astep+D%3A+Does+it+lay+eggs%3F+%7C+Platypus+%7C+Red+kangaroo%0Astep+E%3A+Does+it+have+dry%2C+scaly+skin%3F+%7C+Saltwater+crocodile+%7C+Green+tree+frog%0Astep+F%3A+Does+it+have+six+legs%3F+%7C+Bull+ant+%7C+G%0Astep+G%3A+Does+it+have+eight+legs%3F+%7C+Huntsman+spider+%7C+Garden+snail%0Aspecimen%3A+Red+kangaroo+%7C+A+backbone%2C+fur%2C+and+its+young+grow+in+a+pouch.%0Aspecimen%3A+Bull+ant+%7C+No+backbone%3B+a+hard+outer+skeleton+and+six+legs.%0Aspecimen%3A+Saltwater+crocodile+%7C+A+backbone%2C+dry+scaly+skin+and+four+legs%3B+lays+eggs.%0Aspecimen%3A+Platypus+%7C+A+backbone%2C+fur+and+a+flat+bill%3B+lays+eggs.%0Aspecimen%3A+Garden+snail+%7C+No+backbone%2C+a+soft+body+and+a+shell%3B+no+legs.%0Aspecimen%3A+Kookaburra+%7C+A+backbone%2C+feathers+and+a+strong+beak%3B+lays+eggs.%0Aspecimen%3A+Green+tree+frog+%7C+A+backbone%2C+smooth+moist+skin+and+four+legs%3B+lays+eggs+in+water.%0Aspecimen%3A+Huntsman+spider+%7C+No+backbone%3B+eight+legs+and+two+body+parts.%0Aneed%3A+5%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-7/keys/?full=step-2#step-2)

<!-- Speaker note: Choose a specimen and read its description carefully. Answer each question about **that** specimen. The name appears only when you get it right. -->

<!-- Speaker note: Teacher: insist on reading the description before each answer; ask a student to narrate one path aloud. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Answer two-way questions about features you can observe, one at a time. Each answer sends you to the next question, until you reach a name.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Two ways

**Why is it called a dichotomous key?**

- Each step splits into exactly two choices
- It identifies two organisms
- It uses two kinds of feature

* Answer: Each step splits into exactly two choices

<!-- Speaker note: Dichotomous means dividing into two. -->

<!-- Speaker note: Teacher: the crocodile question is the wrong-turn reasoning; the good-question item previews node 4. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Wrong turns

**A student identified the crocodile as a green tree frog. Where did they most likely go wrong?**

- At "Does it have a backbone?"
- At "Does it have dry, scaly skin?": they answered no
- The key is wrong

* Answer: At "Does it have dry, scaly skin?": they answered no

<!-- Speaker note: A wrong answer at one step sends you down the wrong branch, and every later answer can't fix it. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Good questions

**Which is a good question for a key?**

- Does it have six legs?
- Is it big?
- Is it scary?
- Is it a kangaroo?

* Answer: Does it have six legs?

<!-- Speaker note: A good question is about an observable feature with a clear yes or no. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Mix it up

**Looking back at why we classify: what job is a dichotomous key for?**

- Giving a new organism its name
- Identifying an organism you do not know
- Showing exactly how organisms are related
- Deciding which organism is better

* Answer: Identifying an organism you do not know

<!-- Speaker note: A key identifies an organism by asking about features you can observe. Different classifications suit different purposes. What this step is for: A key is a classification tool for a purpose, and the purpose it serves is identification. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: a different key

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-7%2Fkeys%2Fstep-5&code=widget%3A+key%0Astep+A%3A+Does+it+have+legs%3F+%7C+B+%7C+Garden+snail%0Astep+B%3A+Does+it+have+more+than+four+legs%3F+%7C+C+%7C+D%0Astep+C%3A+Does+it+have+six+legs%3F+%7C+Bull+ant+%7C+Huntsman+spider%0Astep+D%3A+Is+its+body+covered+in+feathers%3F+%7C+Kookaburra+%7C+E%0Astep+E%3A+Does+it+have+fur%3F+%7C+F+%7C+G%0Astep+F%3A+Do+its+young+grow+in+a+pouch%3F+%7C+Red+kangaroo+%7C+Platypus%0Astep+G%3A+Is+its+skin+smooth+and+moist%3F+%7C+Green+tree+frog+%7C+Saltwater+crocodile%0Aspecimen%3A+Red+kangaroo+%7C+A+backbone%2C+fur%2C+and+its+young+grow+in+a+pouch.%0Aspecimen%3A+Bull+ant+%7C+No+backbone%3B+a+hard+outer+skeleton+and+six+legs.%0Aspecimen%3A+Saltwater+crocodile+%7C+A+backbone%2C+dry+scaly+skin+and+four+legs%3B+lays+eggs.%0Aspecimen%3A+Platypus+%7C+A+backbone%2C+fur+and+a+flat+bill%3B+lays+eggs.%0Aspecimen%3A+Garden+snail+%7C+No+backbone%2C+a+soft+body+and+a+shell%3B+no+legs.%0Aneed%3A+3%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-7/keys/?full=step-5#step-5)

<!-- Speaker note: What this step is for: Two keys for the same animals reach the same names by different questions, showing that the features, not the order, carry the identification. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Same names

**The two keys ask different questions in a different order, but give the same names. Why?**

- Each animal has the same features whichever order you check them in
- One of the keys must be wrong
- Keys always ask the same questions

* Answer: Each animal has the same features whichever order you check them in

<!-- Speaker note: A key is one way of checking features. Different keys can sort the same organisms in different orders. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: a broken key

**A key's first question is "Is it brown?" The specimens include a brown kangaroo, a grey kangaroo and a brown spider. What is wrong with this question?**

- Colour varies within a species and isn't reliable: the two kangaroos go different ways
- It should ask about size instead
- Nothing: colour is observable

* Answer: Colour varies within a species and isn't reliable: the two kangaroos go different ways

<!-- Speaker note: Good keys use features that don't vary within a kind of organism: legs, body covering, backbone. What this step is for: Spotting a flaw in a key (a question that doesn't split cleanly) prepares students to build keys of their own. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- A **dichotomous key** identifies an organism with a series of **two-way choices** about **observable features**.
- Answer each question about the specimen in front of you; each answer leads to the next step, until you reach a name.
- One wrong answer leads to the wrong name: check each feature carefully.
- Good questions have a clear yes or no and use features that don't vary within a kind (legs, covering, backbone), not colour or size.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Why must each question in a dichotomous key be about an observable feature with a clear yes or no?**

* So that anyone checking the specimen gets the same answer and reaches the same name.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: constructing keys (node 4), the same logic in reverse: choosing features that split a group cleanly in two. In class: a key to the leaves on the school grounds, or to a tray of mixed hardware (screws, nails, bolts) as a warm-up. -->