For each flip - book does the volunteer think the circles move to the bottom of the card, then back to the top, and so on? (scientificamerican.com)
Observations and results In the first flip - book (which skipped no line spaces), did it look like the circles moved to the bottom of the card, then back to the top, and so on? (scientificamerican.com)
Continue drawing circles on each card, making them four line spaces down from the previous circle, until you reach the bottom of a card. (scientificamerican.com)