Driving carpool is always a surefire way to hear some preschool pearls of wisdom.
Our neighbor's son, S was talking to my daughter Ann (both 4).
S: The last time (another boy) was at my house for a playdate, my brother N was zero years old.
Ann (while wrinkling nose): What's zero?
S (shrugs): I'm not sure, but it means that you don't have any teeth yet.