Piston

Pistons are a hazard present in Portal, though are also sometimes required to be used as more of a game mechanic to continue through the chamber.

Portal
Portal did not feature pistons until late in the game, during Chell's escape. In Portal, pistons serve primarily as a hazard, as a player who is caught between a piston and a wall will be crushed and killed. That said, there are times where Pistons are instrumental to a player's success, such as providing a bridge in Escape 1 or propelling the player through a portal in Escape 2.

Two types of piston are present in Portal, a cylindrical piston and a rectangular piston. The rectangular pistons are only seen once and are used to elevate Chell in order to for her to create a portal on an otherwise unreachable surface.

Portal 2
Pistons do not make any appearance in Portal 2. Their purposes are fulfilled through crushers where the pistons would be hazards and panels where the pistons assisted the player's progress. Their lack of appearance is likely due to the progression of the game, as