Few things in life inspire the sort of jaw-dropping amazement as witnessing someone smash bricks with his bare hands for the first time. I'd always wondered how the person's hands weren't shattered along with the bricks, and now a physics professor explains why in this video.
"It has to do with the impact time. It's very quick," says Richard Brandt, former physics professor at NYU. At the point of impact, the amount of force your hand applies to the brick produces an equal reaction force from the brick to your hand. Your hand is a bit more elastic and can "move up" without breaking. The key is to keep the impact time as short as possible by striking fast and hard while exerting the maximum possible force.
That said, Brandt's explanation takes into account your confidence level. If you hesitate, you'll slow down and increase the likelihood of breaking your hand from a failed attempt. That's why it takes years of practice and focus to break bricks, and why most people who can start with things like boards of various thicknesses.
Seriously, don't try this without supervision unless you know what you're doing.
The key to shattering breaks without breaking your hand [Business Insider]