On Exploration and Play:

"Exploration and play with novel, inanimate objects decreases relatively rapldly, as an animal evidently becomes more familiar with the potentials of the object and as novelty wears off. More complex objects will be the focus of longer periods of attention (Welker 1956a. 1956b. 1956~). Conspecifics probably continue to elicit play because their complexity and thelr almost Infinite behavioral (play) repertoire renders them almost infinitely novel."

Suzanne Chevalier-Skolnikoff