What is the difference between good and bad fats?

If you look at the way people eat and the length of time they live in the Mediterranean, this gives rise to a thing called the Mediterranean paradox, or the French paradox. People there seem to live a lot longer than they ought to because they eat very fatty food. But the fats that they do eat are of a certain type, things like olive oil. Olive oil is very rich in a type of fat called mono-unsaturated. When we talk about fats we talk about long chains of carbon atoms linked together. You can either have one bond between one carbon atom and the next, or sometimes you can have a double bond. And if you have lots of single bonds between them, then that's saturated fat, and it's bad for you. And the reason it's bad for you is that all the carbon chains can get very close together and stack up very neatly. This forms a very solid block of fat. It's not very chemically exciting and it clogs your arteries up. If you have things like olive oil, they have a double bond, which gives the chain a kink. So when you try to press oil molecules together they don't stack up very neatly. They don't form a solid block of lard, they're much more chemically exciting and this is why they're better for you.