Important warning! NOT A JOKE! Pay attention to this.
1. A plastic bottle with a cap (like a normal water bottle).
2. A little Drano.
3. A little water.
4. A small piece of foil.
5. Disturb it by moving and BOOM (Less than 30 seconds)!
6. No fingers left and other serious effects to your face, eyes, etc.
People are finding these bombs in mailboxes and in their yards, just waiting for someone to pick it up intending to put it in the trash. It takes about 30 seconds to blow after you move the thing.
See "SNOPES", it's true: http://www.snopes.com/crime/warnings/bottlebomb.asp