This is the most common type of piston trap. It uses a piston to push a block into a 1x1 space where a mob is standing.

Materials Required:

Steps:

How it works: When a player (or mob) steps on the pressure plate, it sends a signal. The Redstone Torch turns off, causing the Sticky Piston to retract. The player falls into the hole. When they step off the plate (trying to jump), the piston extends again, crushing them against the ceiling or trapping them inside.

This is the most famous design on the Lovely Craft community forums. It is a 2x2 floor suffocation trap.