This is played in a mixed group, with everyone seated in a circle. One player is selected to be "it", and stands in the center of the circle. He/she goes and sits on the lap of someone in the circle and says to them, "I love you, I love you, won't you please smile?" The seated person has to say, "You know that I love you, but I just can't smile", without smiling! If the seated person smiles, they become "it", and start over, choosing someone else to sit on.
One variation, is that "it" can do whatever it takes to get the person to smile; whisper in their ear, pull faces, etc. Playing this game with teenagers or preteens is a riot!