Portals

From the Portal Wiki

Portals are the primary gameplay mechanic of the Portal series. Portals can only be placed on white, flat surfaces by a Handheld Portal Device (simply referred to as a Portal Gun), and works in pairs thereby allowing two-way movement through each portal.

A curious P-body testing and playing with the Portal Gun.
Momentum, a function of mass and velocity, is conserved between portals. In layman's terms: Speedy-thing goes in, speedy-thing comes out.
GLaDOS

Overview

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Pre-release designs

References