Portal consists primarily of a series of puzzles that must be solved by teleporting the player's character and simple objects using "the Aperture Science Handheld Portal Device", also referred to as the "portal gun", a device that can create intra-spatial portals between two flat planes. The player-character, Chell, is challenged and taunted by an artificial intelligence named GLaDOS (Genetic Lifeform and Disk Operating System) to complete each puzzle in the Aperture Science Enrichment Center using the portal gun with the promise of receiving cake when all the puzzles are completed. The Source Engine's physics system allows kinetic energy to be retained through portals, requiring creative use of portals to maneuver through the test chambers. This gameplay element is based on a similar concept from the game Narbacular Drop; many of the team members from the DigiPen Institute of Technology who worked on Narbacular Drop were hired by Valve for the creation of Portal, making it a spiritual successor to the game.