A tornado tube is a fun toy which allows you to create a mini-tornado. You connect two pop bottles together. One of them you fill with water. You then orient the bottles so that the water filled bottle is on top and the empty bottle is at the bottom. You then move the top bottle around in small quick circles, for a few seconds, and what happens next is that the water forms a vortex in which the water spirals down into the bottom bottle. This looks very much like a tornado.
Check out a video of a tornado tube:
Moving the top bottle around in circles causes the water inside to start circling which then becomes a self-sustaining process which continues until the bottle is emptied. If you don't move the bottle around in circles, the water won't form a vortex and as a result it won't empty into the bottom bottle. The water would remain in the top bottle due to the air pressure in the bottom bottle which "pushes" up on the water. But when the water forms a vortex, air is allowed to enter the top bottle through the empty "hole" created by the center of the vortex. This allows the water to easily fall into the bottom bottle.
You can add food coloring to the water which creates a nice visual effect. You can also add cooking oil to the water. The oil, being lighter than water, will stay in the center region of the vortex. This is the same basic effect as oil floating on water. The heavier water displaces the oil and causes it to float on top of the water. In the vortex, the heavier water will move towards the outside, due to centrifugal forces, and this causes the lighter oil to accumulate in the center region.
The tornado tube is similar to a real-life tornado. A real-life tornado forms when air in the atmosphere begins to rotate. This rotation is begun when warm and cold air streams encounter each other in the atmosphere, and due to the difference in densities of these air streams they can combine in such a way that the air rotates. It is this rotation which initiates a tornado.