An experimenter visits a preschool with a big box of markers and paper. The children are told that they can draw as many pictures as they want. The children enjoy the task very much. On another visit, the children are told that for every picture they draw they will earn a prize. Two weeks later when the experimenter returns, he offers the markers and paper for drawing but says that no prizes will be given. The children draw very little with the markers because extrinsic rewards can undermine motivation.