High Energy Pellet

The High Energy Pellet is a puzzle mechanic introduced in Portal. It consists of a launcher built into a Test Chamber surface that emits a fast moving energy ball. It is always accompanied by two receptacles, one that fires the Energy Pellet, while the other catches it. Players had to direct the ball into the receptacle in order to activate lifts or devices before it dissipates and explodes. The Energy Pellet "refreshes" every time it passes through linked portals, meaning the player could keep the ball bouncing until they've discovered the solution to the current chamber. The ball is extremely volatile, meaning anything it comes in contact with that isn't a wall is vaporized, killing Chell and turrets instantly.

Trivia

 * The model used for the Energy Pellet is actually the Combine Energy Ball re-used from the Half-Life series.