Answer:
When they are dribbling
Step-by-step explanation:
This riddle/joke makes sense because when a basketball player moves with the ball legally, it has to bounce it up and down, or dribble the basketball. The definition of dribbling in sports like soccer, hockey, and basketball is, "take (the ball) forwards past opponents with slight touches of the feet or the stick, or (in basketball) by continuous bouncing.
This relates to the baby's action of dribbling, which is most commonly known as drooling. Which is why a baby is like a basketball player when they are dribbling.
I hope it helped!