U Dum is a game where you, the player, tries to get the attention of another player in a variety of ways. After gaining their attention, you immediately shout "U Dum!" However, the defending player may preemptively shout "U Dum" after you have gotten their attention, nullifying your initial attempt.
Todd: "Hey Blair, I learned something interesting about you this weekend."
Blair: "Really? What's that?"
Todd: "I learned that U Dum!"