Humans don’t have a monopoly on horsing around. Animals of all sorts use play to prepare for real-world situations—but their shenanigans can look pretty different from ours. You might catch baby rats mischievously battling to figure out how to fight, or Komodo dragons sticking their heads into buckets to learn about communal feeding. It’s also easy to anthropomorphize and misinterpret actions as gestures of joy that are actually signs of distress or just indifference. So biologists have to develop rules to try to establish what’s play and what’s not. This Venn diagram plots five basic criteria used in animal behavior research to reveal which critters are goofing off for real.