For children, playing with toys is both fun and educational. It helps improve their hand and eye movements as well as develop hand skills to accomplish tasks. It is the same with animals. Young chimpanzees improve their hand skills while playing with sticks. Birds also play with things in ways that teach. Swallows drop and recover feathers in midair. Diving suddenly to catch the feathers is a good practice for catching flying insects.

Social play helps animals learn how to behave when they run into other animals. And it teaches animals about communication. An animal gives signals to let another animal know it wants to play. Many animals give signals by showing a " play face". In some ways this face is like a human smile; the animal holds its mouth in a relaxed and open manner. During play-fights, some animals may step backwards to signal that they are playing and not acting seriously.

Until now, scientists have understood playful acts help prepare animals for their serious adult situations. But the numbers of playful species and examples of animal play are vast. The purpose of animal play may have meanings scientists have not discovered yet.

31. The main purpose of the text is to ______.

A. ask us to take good care of wild animals

B. encourage us to learn more about animals

C. entertain us with interesting animal stories

D. show us something about animal behavior

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