To study play behaviors in animals, scientists must be able to authentically simulate play-conducive environments in the laboratory. Animals like rats are less inclined to play if they are anxious or restrained, and there is minimal data on the brain activity of rats that are free to play. After getting rats comfortable with a human playmate, tickling them under controlled conditions, then measuring the rats' squeaks and brain activity, a research team reports that a structure in rat brains called the periaqueductal gray is essential for play and laughter.