system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can some people smash through concrete blocks without breaking their hands?
gpt
Simple physics.

The force of the smash must go somewhere. It is transferred from the hand to the block, and if there is enough force it breaks causing more of the force to distribute into the "breaking" of the block.

If you fail to break the block, then the force has nowhere to go but into your hand.  If the block is hard enough (or say if you stack enough blocks together), and you don't break them, then you CAN break your hand.

Same principle applies to how cars are designed.  Notice how in a lot of head on collisions with modern cars, the car will completely crumple.  This is by design, because the crumpling of the car absorbs the force of the impact. 

Old cars made of solid steel would not crumple.  So in a head on collision, the force of the impact would get transferred to its occupants, causing nasty results.  The crumpling effect is similar to the block breaking.