一般以為很明顯的概念，不一定很容易定義？
例如，人人以為“玩”很簡單，不過什麼是“玩”？如何定義“玩”？並非易事。
動物為什麼“玩”？
哪些動物“玩”？
果蠅“玩”不“玩”？
“玩”需要符合
沒有立即的生存意義
自願、有意、獎賞
非文化特異
重複但非刻板
無緊張壓力
為什麼要研究果蠅的玩？
因為科學家要玩。
Criteria for play-like behavior require the activity to be (1) of no immediate relevance for survival; (2) voluntary, intentional, and rewarding; (3) non-ethotypical; (4) repeated, yet unstereotyped; and (5) free from stress.Play-like behavior following these rules is pervasive across the vertebrate subphylum； recent studies in rats even identified and characterized the involved brain regions. In invertebrates, sparse reports have so far addressed either social play in parasitoid wasps or spiders,object play in bumblebees, or are anecdotal.1 Contrary to social play or object play, which are thought to train social interaction or muscle/motor skills, respectively, a convincing hypothesis of the adaptive value of voluntary passive movement play-like behavior is currently lacking in any organism.
