When they dribble.
Explanation: This joke plays on the double meaning of the word “dribble.” In basketball, dribbling refers to bouncing the ball while running or moving, which is a fundamental skill. However, “dribble” can also refer to a baby drooling, as infants often do when they’re teething or simply being babies. The humor arises from this clever wordplay, creating a funny connection between babies and basketball skills.