Glucose is a type of sugar that living things use as their main source of energy. Your body breaks down the food you eat, especially foods like bread, fruit, and pasta, into glucose. Then your blood carries this glucose to every cell in your body, kind of like a delivery service bringing fuel to millions of tiny factories.

When you feel hungry or tired, it can sometimes mean your blood glucose levels are running low. Your brain especially depends on glucose: it uses a lot of the glucose in your blood, which is one reason it can be hard to think clearly when you haven't eaten in a while.

Your body has a clever system for managing glucose levels. When you eat, glucose enters your bloodstream, and a hormone called insulin helps move it into your cells, where it can be used immediately or stored for later. Athletes sometimes talk about loading up on carbs before a big game because carbohydrates break down into glucose, giving them energy when they need it most.

Plants make glucose through photosynthesis, combining sunlight, water, and carbon dioxide. In this way, glucose connects all living things: plants create it from the sun's energy, and animals get it by eating plants (or by eating other animals that ate plants).