How young are you that you dont know what these things are for? A human arm? Thats like DUH!!! You have TWO of them. What do you use them for? And cats, surely youve seen cats. What do they use their legs for? Surely you know what bats and dolphins are as well. Picture them in your mind or find pictures. If you cant figure this out on your own them you and a wood post have a lot in common. I mean SERIOUSLY!!
Here are some choices to choose from: eat, swim, jump, fly, run, fight, hold, walk,
The function of a human arm is to move things and pick things up - basically help you to get what you need to get done, the function of a cat leg is to help the cat walk, the function of a dolphin fin is to help the dolphin swim, and the function of a bat wing is to help the bat fly.