High Energy Pellet

The High Energy Pellet is a puzzle mechanic introduced in Portal. It is a fast moving energy ball, fired by an emitter built into a Test Chamber surface and usually accompanied by a receptacle that catches it. Players direct the ball into the receptacle in order to activate lifts or devices before it dissipates and explodes. The ball is extremely volatile, and as such instantly vaporizes delicate constructs like Chell and Turrets on contact.

Trivia

 * The Energy Pellet re-uses the Combine Energy Ball model from the Half-Life series.
 * The 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.