Visual Cliff

The visual cliff is a test given to infants to see if they have developed depth perception. The way it works is there is a platform that is covered with a cloth that is draped all over the place (on the platform, down to the floor, all over...). Then, a piece of glass or other clear material is placed on top of the platform and extends well off of the platform, creating a sort of bridge. An infant is then placed on the platform, and the infant's mother stands on the other side of the clear bridge. The mother calls for the child who, if it crawls off the platform and onto the clear bridge, it does not yet have depth perception. If it stops when it gets to the edge of the platform, looks down, and either is reluctant to cross or refuses to cross, then the child has depth perception. The reason is that the end of the platform looks like a cliff (the child doesn't yet understand that there is some kind of bridge there) and going off the edge of the platform would have bad consequences.