In "Beyond the Pleasure Principle" (Standard Edition, Vol. 18, pp. 14-15), Freud wrote:

This good little boy, however, had an occasional disturbing habit of taking any small objects he could get hold of and throwing them away from him into a corner, under the bed, and so on, so that hunting for his toys and picking them up was often quite a business. As he did this he gave vent to a loud, long-drawn-out "o-o-o-o," accompanied by an expression of interest and satisfaction. His mother and the writer of the present account were agreed in thinking that this was not a mere interjection but represented the German word "fort'" [gone]. I eventually realized that it was a game and that the only use he made of any of his toys was to play "gone" with them. One day I made an observation which confirmed my view. The child had a wooden reel with a piece of string tied around it. It never occurred to him to pull it along the floor behind him, for instance, and play at its being a carriage. What he did was to hold the reel by the string and very skillfully throw it over the edge of his curtained cot, so that it disappeared into it, at the same time uttering his expressive "o-o-o-o." He then pulled the reel again by the string and hailed its reappearance with a joyful "da" [there]. This, then, was the complete game disappearance and return. As a rule one only witnessed its first act, which was repeated untiringly as a game in itself, though there is no doubt that the greater pleasure was attached to the second act. The interpretation of the game then became obvious. It was related to the child's great cultural achievement -- the instinctual renunciation (that is, the renunciation of instinctual satisfaction) which he had made in allowing his mother to go away without protesting.