Camper 1 is on the invisible bench already
Camper 2: (walks up to Camper 1)What are you doing?
Camper 1: Sitting on this here invisible bench.
Camper 2: Can I join you?
Camper 1: Sure!
Repeat until all but one of the Campers are sitting on the invisible bench.
Last person: what are you doing?
All: sitting on this invisible bench.
Last person: How can you? I moved it over there yesterday! (pointing somewhere else.)
All the campers sitting fall down.