The phrase "What did the dentist say to the golfer?" is a popular riddle. The answer to the riddle is "Fore!" which is a term used in golf to warn other golfers of an incoming golf ball.
The riddle is a play on words, as "fore" can mean both "in front of" and "a warning." In the context of the riddle, the dentist is telling the golfer to "fore"warn them of an incoming golf ball.
