- two candybars
Place a candy at each side of the room. Have two people come up and link arms, back to back.
Tell them they have 10 seconds to get their respective candy bar. When you say go they will each begin pulling the other one in the opposite direction, one will either get their candy bar or neither will.
The point is that had they worked together and cooperated to go to one candy bar at a time, taking turns (been unified) they both could have gotten their reward.