Something to mark the boundaries
One person is in the middle of the circle. He walks up to someone in the circle and says, “Bibbity Bibbity Bop”. That person must respond with “Bop!” before the person in the middle says “Bop”.
If the person in the circle is too slow, or doesn’t say “Bop” at all, he switches places with the person in the middle.
The person in the middle can also try and trick the other person by saying things like “Hibbity Hibbity Hop” or “Bop! Say Bop!” If the person in the circle says “Bop” when he shouldn’t, he is in the middle.
Meanwhile, everyone else in the circle is busy winking and gesturing at each other to try and switch places without getting caught. If the person in the middle sees that people behind him are trying to trade places, he can run to an empty space and take someone’s spot.