 Look at this juicy burger! Doesn't it look appetising? It has so many good things in it that my body needs. Protein and fats in the meat and cheese, carbohydrates in the bun, vitamins, minerals and fiber in the salad. But how can I use them? I need to get them into my blood and this burger is never going to fit. That's where digestion comes in. Digestion is the process of breaking down food to make it soluble in the blood. You can digest food in two ways, mechanically and chemically. When I put this burger in my mouth I do both types of digestion. My mouth breaks it down mechanically using my teeth. This mashes up the food into smaller pieces. My saliva carries out chemical digestion. It contains an enzyme called amylase. That takes the big starch molecules you find in the bun and breaks them into smaller pieces called maltose. However, this is just the beginning of the digestive journey and a lot more needs to happen before this burger is going to be small enough to be absorbed into the blood and taken around to all my hungry cells that need the nutrients. The next stage of digestion are summarized in the next videos of the journey. Humans carry out internal digestion as the food is digested inside our bodies. However, other organisms digest the food outside their bodies. Fungi, for example, are called saprotrophs. This means that they secrete enzymes out of their cells onto food around them and then absorb the nutrients once they have been broken down. Houseflies do a similar process where they secrete a solvent onto the food and then suck up the nutrients. Lovely. Imagine spitting on your burger until it's just a soup and then eating it, yuck. Anyway, let's get back to humans. The whole process of digestion can take a long time. It takes six to eight hours for food to pass through the stomach and small intestine and almost 40 hours before you see the remains of your burger coming out the other end. Probably the most amazing thing about digestion, however, is that a lot of it isn't actually done by ourselves, but by 100 trillion bacteria living inside our guts. They help break down certain foods for us, make vitamins and help with the immune system. So there you go. With the crushing some enzymes and some friendly bacteria, you can turn this juicy burger into small soluble molecules that can be carried around in the blood.