An air cannon

What you can do here
Strike the rubber surface of the cannon with your hand. Watch the objects in front of the cannon. To divert the shot in vertical direction, lift or lower the barrel, simultaneously pushing the black button. If possible, stand in front of the cannon several meters away and ask another person to “fire”.

How it works
Striking the rubber surface pushes the air out of the barrel through the hole. On the edge of the hole, friction occurs which makes the air layer closest to the edge rotate. As a result, an air ring or a whirlpool is formed, similar in shape to the popular Krakow bagel. A ring of whirlpools is a very stable structure and it moves in the air pretty much like a solid body.

Interesting facts
Smoke rings can sometimes be formed by active volcanos. Similar rings formed underwater, are often used by dolphins to communicate with one another.