Rufus Hound, on an old episode of The Infinite Monkey Cage, told a tale of his time working at the SCIENCE Museum. Take a suitable plastic bottle and add liquid nitrogen — the suitability of the bottle being judged by its ability to not quite explode immediately. Hand the bottle to your enemy and watch with glee as the warmth of his hand boils the contents of the bottle until it can no longer withstand the internal pressure. Run away.