A hormone is a chemical your body makes to signal it's time to do something important. It acts like a traffic light in your body. Hormones signal your cells and organs when it's time to go to work, when it's time to slow down, and when it's time to stop doing something.
When you need energy to play and learn, some hormones tell your stomach it's time to be hungry. After you've eaten, other hormones signal that you've had enough. Hormones help make you sleepy and remind you that your body needs to rest. Once you've rested plenty, a different set of hormones help you wake up and feel alert in the morning.
Hormones are very important to growing. Some tell your bones and muscles to get bigger. Others signal the start of puberty, the process of developing a grownup body. You have lots of these hormone chemicals in your body, and they all help your cells and organs work together.
(by Ali Rodenhauser)