Play is something most of us associate with higher-order animals typically mammals – we’re ok with the idea of cats and apes playing, but not snakes, for instance. While fish certainly sit lower on the cognitive totem pole than many animals, one study by researchers at the University of Tennessee suggests that some species engage in “play” activity that lacks any function other than plan old fun.

“Play is repeated behavior that is incompletely functional in the context or at the age in which it is performed and is initiated voluntarily when the animal or person is in a relaxed or low-stress setting,” said Gordon Burghardt, a professor in the departments of Psychology and Ecology are Evolutionary Biology. His unique (and possibly contentious) definition of play allows researchers to define and observe it in species you wouldn’t otherwise suspect.

Burghardt and his team studied three male specimens of a species of chichlid fish (specifically the white spotted chichlid) over a period of two years. Over that time, they observed repeated instances of the fish bumping into a weighted thermometer that would right itself due to being bottom-heavy – not unlike the bottom-weighted inflatable punching bags you may have enjoyed as a child.

The behavior appeared to be unprovoked. Food was not used as a reward, and bopping the thermometer had no meaningful impact on the fish or its surroundings. The three subjects were observed independently, so it couldn’t have been a learned behavior. It appears that for all intents and purposes, the fish were knocking into the object because knocking something over (and then watching it pop back up) is a fun thing to do.

“The quick righting response seemed the primary stimulus factor that maintained the behavior,” said Burghardt. “We have observed octopus doing this with balls by pulling them underwater and watching them pop back up again. This reactive feature is common in toys used for children and companion animals.”

Burghardt believes his definition of play demonstrates that it’s not just fun, but woven into the fabric of animal existence; part of animals’ evolutionary history.