Articles Required : An empty cardboard shoe-box, two chimneys of kerosene lamp, candle, cigarette and matchstick.
Whenever you burn something the smoke moves upwards. Isn't it? Now, how would you react if you're asked to make it go downwards? Will you be able to do it? Well, once you try the experiment explained below, you'd naturally quip after that "Oh, it was so easy!"
Make two holes in your shoe-box as shown in the figure here and fix the chimneys in both of them. If you can't find such chimneys, you can use even the empty talcum powder tins after cutting their tops and bottoms. Having done this, remove the shoe-box cover and put a burning candle just below the left chimney. Now replace the cover and paste cello-tape on the joint of box and its cover to restrict air entry from the slit.
Now, if you light a cigarette and bring it on top of the second chimney, something interesting would happen. The cigarette smoke, instead of going upwards, would start travelling downwards. Isn't that strange?
Let's explain the logic of it. You know very well that the hot air is lighter than the cool air. The air above the candle becomes hot, moves upwards, and escapes from the left chimney. Now, this gap has to be filled and as there is no inlet for the air except the right chimney, the air comes in through it, creating a force of suction which draws the cigarette smoke alongwith it. So instead of going up smoke goes downwards.