I've started to learn about fermented foods. At the moment all I do is sauerkraut and sourdough but I can tell it's something that will be with me for life now so I'm bound to learn more and more.
For those who don't know: right now there are millions of microscopically-tiny creatures all around you, swimming around in the air, floating up your nose, kicking about on your skin and just generally having a laugh. Fungal spores ('seeds' of mould, mushrooms and so on) and bacteria are always around us, they are as essential a part of the ecosystem as plants and animals. Inside our guts they are an important part of our digestive system - if we didn't have them we'd not be able to digest our food properly and just having the wrong balance of one variety over another is quite likely to cause stomach and digestion problems. It's like a garden, if you let it get over-run with nettles it's no fun.
One of the most well known fermented foods is bread. Yeasts (there are lots of different kinds) feed off the carbohydrates in flour and excrete gases (fart, basically) which produces bubbles and makes dough rise. Nowadays bread tends to be made with commercial yeasts but originally (and in the case of sourdough) it was/is made using natural yeasts and bacteria found in the air around us. If you mix flour and water and let it sit, these microorganisms will settle in it and get to munching. With a little control over conditions (mainly temperature) and repeated additions of flour and water, over a few days you'll end up with a frothy, bubbling, soupy creature called a sourdough starter. You now have a pet which needs to be fed every few days if you want to keep it alive (they can live for a century or more and are often passed down through generations). In return you'll be able to bake bread, pizza, make pancakes and all other kinds of rising baked goods.