The Eye Wall
An Eye Wall is a white wall with a strange eye embedded on it. They react when a player looks at them for an extended period of time. At first the eyelid moves slightly, then a larger movement, then a blink and finally the wall disappears entirely.
The time required to activate an Eye Wall differs in different puzzles. The Eye Wall in Leap Of Faith takes as long fifteen seconds to activate, whereas the Eye Wall in Don't Look Down takes only half a second. All Eye Walls go through the same motions, but at a different speed. Some Eye Walls will reappear after an unrelated length of time.