An occlusion event tests the knowledge that an object exists even if it is not immediately visible. Jean Piaget originally called this concept object permanence. When Piaget formed his developmental theory in the 1950s, he claimed that object permanence is learned, not innate. The children's game peek-a-boo is a classic example of this phenomenon, and one which obscures the true grasp infants have on permanence. To disprove this notion, an experimenter designs an impossible occlusion event. The infant is shown a block and a transparent screen. The infant habituates, then a solid panel is placed in front of the objects to block them from view. When the panel is removed, the block is gone, but the screen remains. The infant is confused because the block has disappeared indicating that he/she understands that objects maintain location in space and do not simply disappear.