Both forms of attention are of real importance in the world of play, but we will note only those cases in which the effort of attending is itself the subject of playful exercise. Sikorski has asserted forcibly that children frequently make use in their play of the expectation of a familiar impression whose memory picture is already present in the mind; what Lewes calls “preperception,” and Sikorski “reproduction préparatoire.” He says: “It is very interesting to notice how children use attention in their play. It is one of the most salient features of all the mental operations of children in all their busyness and destructiveness. It may be called a sort of mental auxiliary which gives variety to play.”[316] He goes on to instance Preyer’s son, who opened and closed the cover of a can seventy-nine times in succession, and evinced the closest attention all the while.[317] The expectation of a resulting sound is no doubt an essential part of such play as this. Alternate stress and relaxation of attention account for the charm of hide and seek. Darwin says that his son on the one hundred and tenth day was delighted when a handkerchief was put over his face or his playfellow’s and then suddenly withdrawn.[318] While surprise was probably the principal cause of this delight at first, on its repetition expectation and the sudden revelation must play a part. When a child throws stones in water or at a mark, batters an old pot, awaits the tossed-up ball or watches a rolling one, we must reckon with the pleasure which is derived from the exercise of close attention, as well as that in movement as such, and in this kind of play the comparison of memory pictures with present reality. “In all such play,” says Sikorski, after instancing several examples, “a particular result is expected and awaited as something desirable. The sound of the stone striking the water, the direction taken by the soap bubble the moment it is tossed off,[319] all such consequences are pictured in advance, and the essence of the enjoyment consists in the coincidence of reality with the mental image.”[320]