And when the results were studied it was worked out that the kids who had eaten cereal got the best results.
Things like chocolate give you a quick burst of energy, but the energy from cereals works more slowly, helping people think for longer.
A different study showed that children aged 9-16 missed breakfast 17 times out of every 100, and when they did eat something, it was often a chocolate bar or fizzy drink.
The breakfast research was paid for by a group called Cereal Partners UK.
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