Your body needs energy to do things. Energy comes from a kind of sugar found in the food you eat. This sugar is called glucose. Glucose travels in your blood. Without glucose you wouldn't be able to study, play, or even eat or think. But if too much glucose builds up in your blood, you can feel sick. This is called high blood sugar (hyperglycemia). High blood sugar happens when you eat too much or don't take enough insulin. It can also happen when you're sick, worried, upset, or excited. It can happen if you haven't had as much exercise as normal. If your blood sugar gets really high, it can be dangerous.