How small are atoms?

My 7th graders are learning about the atom, and that includes how small they are. So I found out how large a mole of water is (18 ml). Then I filled a small flask with that amount of water with food coloring. Then I wrote down how many atoms are in the flask:
1.0866 × 1024 atoms
180,660,000,000,000,000,000,000 atoms
1.8066 septillion atoms
1.8066 yotta atoms
The human brain just isn’t wired to be able to comprehend the scale of these numbers.