Difficulty: Medium

Bobby Braingle is playing a game of golf with his boss. Bobby is not a very good golfer, and on the second hole, he hits his ball into a sand trap. His boss, who is very skilled, hits his ball onto the green. When Bobby arrives at the sand trap, he notices that there are two golf balls in the sand trap. Both are face down so he cannot identify which one is his. What should he do so as not to incur a penalty?