1. An automated method of distinguishing a computer from a human, comprising: displaying on a computer screen an animation comprising at least a first layer and a second layer, one of said first layer and said second layer comprising a recognizable image and the other comprising a partial obstruction that obstructs a portion of the recognizable image and that does not obstruct an other portion of the recognizable image, wherein said animation comprises relative motion between said first and said second layer, the relative motion causing the obstructed portion of the recognizable image to become unobstructed, and the relative motion causing the other not obstructed portion of the recognizable image to become obstructed; receiving a reply to the displayed animation; and determining that the reply originated from a human based on the reply being consistent with an aspect of the recognizable image, wherein the animation further comprises a third layer, said third layer comprising a partial image that is at least partially complementary to said recognizable image provided in an incomplete form.