In order for a tornado to form, a number of different things have to happen. The first thing that has to happen is that you have to have two different patches of air moving toward each other from opposite directions, with a warm one on the bottom and a cool one on the top.
(Pictures all come from ABC News
The next thing that happens is that the warm air rises. (You may have learned about hot air rising in school.) After the warm air goes up, the cool air comes down to take its place. So now the warm and cool patches of air are moving side to side and up and down. So they make a spinning patch of air along the ground. This is probably pretty hard to imagine, so check out the picture:
Usually when a tornado is starting, it happens during a storm. So there's lots of air moving around already. If a draft of air from the storm comes along close to the ground and then moves upwards (called an "up-draft"), it will pull the spinning air upwards.
Now comes the part that we don't understand quite as well. So far the spinning air is still really really wide. Somehow it gets pulled in really tight so that it makes a tornado. (Scientists aren't exactly sure why this happens sometimes but not at other times.)
Thanks for the question!
(republished on 08/01/06)