Exchanger

Exchanger is an arcade game spanning two parallel universes, separated by a barrier. You control an exchanger, which is a device made of black and white holes that will suck aliens through a tunnel from one universe to an outlet in the other. The device will periodically change the direction of transfer after a short pause in operation. You can directly control the motion of one end of the exchanger, while the other end is controlled indirectly and may move in the opposite direction from the first end. Each level is initially populated with aliens of two types that have appeared in the wrong universes. Your aim is to send all of the aliens home, using the exchanger, so that the blue aliens are on the right side of the barrier while the yellow ones are on the left.

If you exchange an alien twice within a short period of time, it becomes angry and turns a red colour. An angry alien will wreck your exchanger if you touch it with the outlet, causing you to lose a life. Later levels also introduce mines which will detonate after contact with either end of your exchanger.