Chef Marc Vetri helps American school battle obesity
27 March 2015 Last updated at 16:04 GMT
All this week Newsround is looking at food, and how you can eat more healthily.
As part of a special programme, Newsround reporter Ricky visited the United States where obesity is a big problem.
In the USA, 1 in 3 children and almost 7 in 10 adults are overweight or obese.
It's become such a big problem that one school in Philadelphia has started trying to get kids to think a lot more about their lunch.
Philadelphia is one of the first cities in the USA to reduce the amount of obese children.
They're being helped by celebrity chef Marc Vetri - America's version of Jamie Oliver.