On a flat tabletop, arrange two empty cans so you can blow between them. Depending on one small detail, the cans either move apart or come together.
Place the cans side by side on a table, parallel to each other and about an inch (2.5 centimeters) apart. Make sure they can roll easily. Now blow straight downward between the two cans (see images below). What happens?
Now try again, but this time, place the two cans on top of two parallel straws. The straws should be arranged perpendicular to the cans and parallel to each other, about 2 inches (5 to 8 centimeters) apart. Blow down between the cans and watch what happens now (see images below).
The cans move apart when placed directly on the table, but move together when resting on the straws. Why?
When you blow down between the cans that are resting on the tabletop, the air is trapped between the cans. As you blow, the pressure between the cans builds up, becoming higher than the pressure on the outside of the cans. This higher pressure forces the cans apart.
Putting straws under the cans allows air to escape beneath the cans. As the air is blown downward, it passes between the cans and gets constricted, speeding up as it squeezes between the cans. When air speeds up, its pressure drops—so says Bernoulli’s Principle. Since the atmospheric pressure on the outside of the cans is higher than the pressure between the cans, the cans roll together.
Try other arrangements of the cans and see what happens. For example, place the cans on the tabletop, as in the first arrangement, but instead of blowing from the top down (vertically), blow between the cans, horizontally, as shown in the photo below. Blow so that your airflow passes along the length between the two cans. Give a good strong burst or puff of air.
Engineers and designers have to take these kinds of pressure differences into account when designing buildings, vehicles, aircraft, and even musical instruments.
This is a great introductory activity for teaching about fluid dynamics and pressure, and for launching a host of design challenges.
Turn this activity into a data-collecting experiment: How far apart can you position the cans and still make them roll together…or apart?