@article{SaporaMitchell1961, author = {A. V. Sapora and E. D. Mitchell}, title = {Explanations of Play and Recreation}, journal = {Sapora, A. V., Mitchell, E. D. (Eds.): The Theory of Play and Recreation, 3rd edition, New York (Ronald Press) 1961, (Chapt. 4) pp. 77-111.}, year = {1961}, }