system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are free radicals and antioxidants?

What do they do that makes them bad/good? Why do some foods have more of the antioxidants?
gpt
All day, you breathe air in and out. This is to get yourself oxygen, a type of atom which is very important to have. 

Your body needs oxygen because of one thing oxygen has, that not many other atoms have. Oxygen is very sticky. It's one of the stickiest atoms there are. Oxygen likes to stick to stuff because of it's "electronegativity", and it will try to stick to whatever it can.

When you breathe in oxygen from the air, it comes in pairs. These are two oxygen atoms stuck together, making their stickyness cancel out. Your body knows how to unstick them and use them to glue other atoms together to form molecules that can be used throughout the body to make things, from bones to skin. But when a single lonely oxygen atom gets free, we call it a "free radical"

A free radical is just a single oxygen atom, but remember how we said oxygen is very sticky? It will try to stick itself onto other molecules, even when it shouldn't! This can easily break things, because the molecules in your body have to be very carefully made. If an oxygen molecule just walks up and sticks to it, this is very bad.

Lucky for us, there's other types of molecules called "antioxidants". Antioxidants are molecules too, but they have a special slot just for sticky oxygen atoms. Antioxidant molecules are sticky too, but they have a shape that only fits with a sticky oxygen atom. If you eat antioxidants, they will go around your body gobbling up all the sticky oxygen atoms, before they stick to something they shouldn't.

They only work on sticky oxygen, they don't work on the paired oxygen you breath in, so don't worry about them using all the oxygen in your body.

Some foods just happen to have a lot of antioxidants. It's because food is made up of molecules too, and some of those have more antioxidants than others. They make them on their own as they grow so they don't get broken by sticky lone oxygen molecules either. 

Your body makes it's own antioxidants too! But sometimes it can't make enough on it's own, so you need to eat them to get more. The healthier you eat, the more antioxidants you can get, and the less likely your body will get hurt by those sticky lone oxygen molecules.