Some kids who have problems with being hyperactive could get worse if they've been guzzling sweets and fizzy drinks.
Scientists have thought for ages that some chemicals, called additives, which are used to make food and drink look or taste different affect kids' behaviour.
Now they say a study of 300 kids proved their theory and loads of you agree additives change your behaviour too.
Ten-year-old Molly from Manchester said: "If I drink too much fizzy drink I go hyper and my cheeks go red!"
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Some kids have problems with something called hyperactivity, which means they can get excited very easily, have loads of energy and can find it difficult to concentrate on things.
That can make it difficult for them to do well at school.
Sacha, 11, emailed us from Naples in Italy to say: "If I eat rubbish like ice cream or cake I get hyperactive. But when I drink too much fizzy I am fine, just feel a little ill."
And 10-year-old Lewis, in Stowmarket, said his mum recently put him on a diet to see if it changes his behaviour.
Government food experts now say some children should have fewer additives.
The scientists' study is one of the first pieces of evidence that proves additives can change the way kids act.
Even so, other experts have said additives don't make children hyperactive in the first place, although they can make the problem worse.