Split the group into half, and place a line that separates them from each other. Then place a rope perpendicular to that line with one half on either side. The challenge for the group is to have their side of the rope all the way on the other side and vice versa, at the end of a given time. After a while of trying to throw the rope back and forth, they might figure out that they can simply hand their side to the other while trading with the other team, but that becomes the challenge so let them figure it out.