The portals can be used in different ways, notably as a way to catapult Chell. If the player cannot put a portal on a high ledge but can put one on a wall across from the ledge and one on the floor below, these portals can be used to propel Chell forward far enough to reach the ledge. This is because portals do not affect forward momentum. When Chell enters the portal in the floor and exits the portal in the wall, she will fall, gaining speed. If she goes through the portal on the floor again while falling, she will be going fast enough that when she exits the wall portal again, she will be able to land on the ledge.