One of the challenges babies face is discriminating separate objects from what surrounds them. Experiments have shown that by 2 months, many infants use movement to help them determine distinct objects. For example, in one experiment infants were shown a rod partially hidden by a block in the middle, so that all the infants could see were the two ends of the rod. When the rod didn't move, infants didn't know whether the rod was a single "whole" rod or whether it was two rods with a block in the middle. When the rod moved behind the block, infants used this information to determine that it was a single rod.