Lots of children grab a cereal bar on the way to school because they haven't got time to sit down for breakfast.
But research has shown these bars could be unhealthy and contain more sugar and fat than you think.
Food watchdog, the Food Commission, found the Kellogg's Coco-Pops bar had more calories in it from sugar than from chocolate - even though it is a cocoa bar.
These cereal bars are also bad for your teeth, they claim.
Even if you heap sugar into your cereal bowl, it tends to be dissolved by the milk, whereas in a bar it can to stick to your teeth.
Out of 18 different bars tested, all of them had higher levels of sugar than is recommended for a healthy breakfast.
But a Kellogg's spokesman said their bars provide "essential vitamins and minerals" and they stick to Government rules about levels of sugar and fat.