All living creatures have several stages of development. After birth, animal and human babies grow. They become adolescents and then adults. But each species develops differently. A zebra can walk about 15 minutes after it's born. At the same time, a baby kangaroo is very small, so it stays in its mother's pouch from 7 to 10 months. Becoming an adult is important for animals and humans. For example, a baby frog has no legs and cannot leave the water until it grows up. People stop growing at different times. Still in most countries they officially become adults at the age of 18. Adults have more rights. They can vote, drive a car and get married. This book compares humans and some animals at different stages of development.