This activity is an introduction to atomic theory and asks students to consider why we should believe in atoms if we can't see them. An analogy is made between a garbage collector who arrives early in the morning and removes the trash but is never seen and the atom, which is never seen, but which has principles that we can demonstrate through simple experiments. Our understanding of atomic theory is necessary for our basic understanding of how elements interact.
I would use this activity at the beginning of a unit on atoms and elements. It can be a warmup activity or a longer lesson with class discussion.