Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Place each animal in its class and say what makes it unusual for that class: Ornithorhynchus, Struthio, Pteropus, Aptenodytes and Crocodilus.
Fishes have 2 heart chambers, amphibians 3, most reptiles 3 (crocodiles 4), birds and mammals 4. Only birds and mammals are warm-blooded. Reptiles have dry skin with scales; birds have feathers, pneumatic bones and air sacs; mammals have hair, pinnae and mammary glands, and most are viviparous, but Ornithorhynchus lays eggs.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, compare your working with the solution.
**Step 1. Ornithorhynchus, the platypus. A mammal, with hair and mammary glands, but it lays eggs**.
**Step 2. Struthio, the ostrich. A bird with feathers, but it cannot fly**.
**Step 3. Pteropus, the flying fox. A mammal, with hair and milk, but it flies**.
**Step 4. Aptenodytes, the penguin. A bird that swims** and cannot fly.
**Step 5. Crocodilus, the crocodile. A reptile, but its heart has four chambers**.
Answer: platypus, a mammal that lays eggs; ostrich and penguin, flightless birds; flying fox, a mammal that flies; crocodile, a reptile with a four-chambered heart.
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