Observer

An observer is a block that emits a quick redstone pulse from its back whenever its "face" detects that the block, fluid, or air directly in front of it has changed.

An observer requires a pickaxe to be mined. When mined without a pickaxe, it drops nothing.

An observer is placed similarly to a piston. It observes the block that it is placed against. The texture of the detecting side is that of an observing face. As observers can detect the state of other observers, placing two adjacent observers, each watching the other, can make a fast and compact redstone clock.