What’s the greatest invention ever? Why?

The application of fire to cook food. I commend whoever utilized fire to cook fish and pork. That's more a discovery than an invention, though (as I've noticed more than half the things in this thread are).

I'm going to go with the printing press as my "greatest invention" simply because I love books and the written word and if the printing press didn't exist, we'd all be waiting around for people to transcribe things for ages or be illiterate.