It's easy to make a fire tornado out of household objects. You just need some lighter fluid, a lazy susan or microwave tray, and a mesh metal garbage can. But why does the flame get so high just from a little rotation? We'll give you a quick physics explanation.
Fire tornadoes look powerful - and I'd advise against messing with one - but they climb so high because the fire is actually having a tough time. It's being deprived of oxygen. This is a rare thing for fire, because it creates the mechanism that brings more oxygen to it. It heats the air around it - air thick with carbon dioxide - and that heat cause the air to become less dense. Less dense air floats upwards, creating a vacuum that suck fresh air in. This is how fires keep feeding themselves more oxygen.
If there's rotation of air around the flame, the air still gets sucked in, but it forms a kind of whirlpool around the flame. Like whirlpools in water, the air goes faster and faster the more it's sucked towards the flame. You can see this principle at work if you spin your office chair and slowly pull your arms or legs inward, or if you twirl an object on a string and pull in the string. As it gets closer to the center, it spins faster.
Even though the air moves fast, it is not providing an efficient oxygen delivery system to the fire. It would be much more efficient just for the air to move directly in. Instead it's forming a swirl that keeps the air circling the fire for quite some time. The fire is choked off, and elements that would have caught fire immediately in an oxygen rich environment climb higher before they get the oxygen they need to completely combust. So the flame moves upwards, and we get a dramatic fire tornado.
[Via The Naked Scientists]