system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can martial artists (I’m thinking karate) chop something like a solid brick in half with just their bare hands?
gpt
There are two components to this; physics and exaggeration.

The first part is the "real" part. Martial artists do actually break things with their hands and feet as a training exercise. If you take a board or a brick and deform it enough it will break. If you hit it with sufficient momentum you'll deform it sufficiently. Getting that amount of momentum is a combination of the mass of the object (for example your hand) and the velocity of that object. So breaking a board is really just a matter of hitting it really hard.

In order to do this with a body part there are a couple of factors that come into play.

Most people don't want to hurt their hand so when they hit a board they'll instinctively pull back at the last second. This reduces the momentum of the hand and then the board won't break. Ironically this tends to hurt more. When you hit a board your hand and the board experience equal and opposite forces. If you actually break the board that force is spread out over the time between when your hand contacts the board and when it breaks. If you don't manage to break the board that force is only spread out over the time it takes the board to bend and bounce back. So your force per time can be much higher and it hurts more. This is why martial artists are trained to "strike through" the target.

The next big issue is aim. If you don't hit the board in the right spot you make it much harder to break. Imagine a board suspended between to holders. It's obviously much harder to break the board if you hit it near one of the holders than if you hit it right between them.

Then there's how you hold your hand. Hands have all kinds of tiny, easily breakable bones. A large part of board breaking training is learning how to hit the board using the strong parts of your hand.

There's also conditioning. If you hit things over and over it not only strengthens your bones and muscles, it also deadens the nerves and makes it hurt less.

Depending on the material it may also be the case that you can't break the board at all, at least not without mangling your hand. Think of a bullet that goes through a board. It may make a hole but the bullet is not going to be bullet shaped when it's done.

Then there's the exaggeration part. Some martial artists just like to show off. There are ways to make a breaking demonstration look much harder than it really is. If you put spacers between boards or bricks they're easier to break since instead of reinforcing each other the weight of the first brick can help break the bricks below them. Not all bricks are created equal either. Some are weak and crumbly. Some bricks are so hard you couldn't break them with a sledge hammer. You'll also notice that people almost always break boards with the grain rather than against the grain.