How are smoke rings formed? Natasha heads to the prep room to test out air cannons. Subscribe for regular science videos: http://bit.ly/RiSubscRibe A toroidal vortex ring is formed when a fast moving fluid moves through a relatively still other or different fluid. For example, smoke rings blown through air, or bubble rings in water. In our cannon example, to start with, the smoke is basically a fast moving ball. As it emerges through the opening, the smoke outside of the ball is being slowed down because of the friction between it and the edges of the hole. Once it leaves the cannon, there is friction at the interface between the smoke and the air in the room. The smoke in the centre of the ball is moving faster than the smoke around the edges, so the smoke on the edges starts to curl around and form a mushroom cloud. As the smoke reaches the back of the cloud, it’s drawn into the faster moving current of air in the centre. It’s this flow pattern that eventually causes the ring to form. The Ri is on Twitter: http://twitter.com/ri_science and Facebook: http://www.facebook.com/royalinstitution and Tumblr: http://ri-science.tumblr.com/ Our editorial policy: http://www.rigb.org/home/editorial-policy Subscribe for the latest science videos: http://bit.ly/RiNewsletter