Recent events in the news might have your kids asking about tornadoes. These destructive storms are caused by a vortex. You can use the classic “tornado in a bottle” demonstration to explain how a vortex forms. This article from Exploratorium explains how to make a vortex using two plastic bottles, a washer, and some tape.
Basically, you add some water to one of the bottles. Then you put the two bottles next together, with the washer in the middle and use some tape to secure the seal. The bottles are then oriented so that the full bottle is over the empty one and the water is dripping down. Rotating the bottles will form the vortex.
What happens is that when the rotating water in the upper bottle drains into the lower bottle, a vortex forms. Gravity pulls the water down, while centripetal force keeps it rotating. You will see a hole in the middle of the vortex. This allows air from the lower bottle to travel into the upper bottle. See the article for a complete explanation.