Have you ever seen someone make smoke rings? They have the scientific name vortices and are related to other vortices like the whirl pool in your tub or sink and even tornados. They look neat and they have some very interesting scientific properties, but since smoking is very bad for you it seems impossible to make them. Here's a way to make smoke rings without smoking.
Take a cardboard box (a shoe box or tissue box work well) that is open on one side only. Cover the open side with some plastic wrap (like Saran wrap) and tape it on well. On the opposite side of the box cut a round hole 3 or 4 inches across. You now have your smoke-ring maker so it's time to load it with smoke. This part can be dangerous so please ask an adult to help. There are many ways to get "smoke" into your smoke-ring maker; you can put a piece of "dry ice" into the machine (don't touch the dry ice with your skin, it is very, very cold), you can use some baby powder, if it is fine enough, or you can use smoke (but be careful that you don't light your box on fire!). You don't need much smoke to make many rings.
Once your machine is loaded you are ready to start experimenting. To make a smoke ring just tap the plastic covered side. What happens when your smoke rings hit something like a stick? Try making two smoke-ring machines and then try sending the rings at each other. What happens when two smoke rings collide? (It's hard to make them collide so you may have to work at it for a while.) Try different methods for making the rings easier to see (like shining a flash light on them). Can you think of more experiments for your smoke-ring machine?