The mirror test is a measure of self-awareness developed by Gordon Gallup Jr. in 1970, that was based in part on observations made by Charles Darwin. While visiting a zoo, Darwin held a mirror up to an orangutan and recorded the animal's reaction, which included making a series of facial expressions. Darwin noted that the significance of these expressions was ambiguous, and could either signify that the primate was making expressions at what it perceived to be another animal, or it could be playing a sort of game with a new toy. There are nine species that pass the mirror test, including magpies and elephants but mostly primates. Most human babies do not pass the mirror test until several months of age. Gordon Gallup built on these observations by devising a test that attempts to gauge self-awareness by determining whether an animal can recognize its own reflection in a mirror as an image of itself.