12. Bulletproof.

How do you stop a speeding bullet? From body armor to armored cars and trucks, we review the history of the race between the bullet and a successful way to stop it. It's not exactly easy to design material that can catch gunfire traveling up to 3,000 feet per second. We'll look at little-known advances like bulletproof layering hidden in walls, futuristic smart materials that "remember" how to stop a bullet, and a system that deploys a shield within milliseconds when it detects an oncoming round.