What Biologists Actually Mean by Play
The word gets used loosely, so the science tightened it. Gordon Burghardt, a biologist at the University of Tennessee, spent years building a precise five-criteria definition: play must be voluntary, pleasurable, structurally similar to serious behavior but exaggerated or incomplete, initiated by the animal itself, and repeated when the animal is well-fed and unstressed. Every criterion has to be met. A dog chasing a ball clears all five. A cat stalking a bird does not, that is hunting, with real stakes.
