We all know we need food and water to grow and develop, but some kids start to worry about food and some almost stop eating altogether.
It's because of an illness called anorexia.
There are no official figures on how many young people are suffering from eating disorders in the UK.
But the charity BEAT believe the number of children developing them is on the rise - the youngest case they found was six years old.
Here, Susan Ringwood, the head of BEAT, explains what you or anyone you know should do if they're suffering from an eating disorder.