Barriers
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Barriers are solid objects that prevent the player and objects from going through. They are commonly used to prevent the player from entering an area but let them interact with the area to some extent.
Barriers
Glass
Glass is used in Portal 2 as well as Portal. It blocks bullets from turrets but allows a Thermal Discouragement Beam to pass through it. Glass is transparent and gels do not stick to it.
Grates
Grates, like glass, allow Thermal Discouragement Beams to pass through. However, they also allow bullets from Turrets to pass through. Gels flow through grates but do not stick to them.
Portal surface
Portal surfaces are white and conduct portals. According to Cave Johnson in Portal 2, good portal conductors are made of concrete and moon rock dust. They are controlled by panels. Gels stick to portal surfaces and do not interfere with their ability to conduct portals.
Regular wall
Like portal surfaces, walls are controlled by panels. It is possible that the wall might be painted to stop it from conducting portals, as in one test chamber things peel off the walls and the wall becomes a portal surface. Gels stick to walls.