The main purpose of enzymes is to break down proteins in foods, so that they can be easily digested, and so that micro-organisms can also be killed in the process of digestion.
So, to the left of me is the enzyme. The enzyme looks a bit like Pacman, and as you can see, it looks like this enzyme has a mouth. This 'mouth' is called the active site, because this is where chemical reactions take place!
Now, to the right of me are these things called 'Substrates'. Substrates are what you get after eating food - they are the result of these enzymes working on the food and breaking down proteins.
Substrates all have different shapes to each other, and so do enzymes, but for every substrate there is an enzyme to go with it. Every substrate goes with an enzyme until the substrate is broken down, OR the enzyme and substrate are joined together with another molecule to create something else.
The merging of the enzymes and substrates is called a catalysis; this is where a substrate has changed. If you look at the way the substrate fits perfectly into the enzyme, this is called a 'lock and key' method.
Enzymes are just like 'locks', they can't be opened without their key - substrates! This is because their specific job is to join with substrates in order to function properly. This process can only be performed in the active site.