Question

read the paragraph. then, add the best topic sentence. some scientists think that yawning makes people feel more awake by bringing more air to the brain. that would explain why people yawn when theyre sleepy or bored. other scientists say there isnt enough proof for that idea, and they suggest a reason for why people yawn when they see or hear someone else yawn. they say yawning at the same time makes people feel closer to each other. there is some proof for this idea, but it wouldnt explain why people yawn when theyre alone. people often yawn when a friend does. scientists arent sure why people yawn. people, dogs, bears, and chimpanzees all yawn.

Explanation:

The paragraph presents different scientists' theories on why people yawn, with no clear - cut conclusion. The best topic sentence should summarize this uncertainty.

Answer:

Scientists aren't sure why people yawn.