The word "kangaroo" comes from the language of the Australian aborigines. It practically means "I don't understand", which was the response of the British James Cook when he was asked what the animal was. What a misunderstanding! These marsupials have a kind of natural "rubber band" on their back legs, which propels them upwards with each jump. The faster the kangaroo bounces, the further it suddenly jumps forward, and the less energy it consumes.