Things you need:

• 4 glass containers that you can balance on one another
• Food coloring
• Warm water
• Cold water
• Playing card

The Experiment: 

Fill two bottles with warm water from the tap and the other two bottles with cold water. Add some blue food coloring to the cold water. Each bottle must be filled to the brim with water.
Cold over hot: Place the playing card over the mouth of one of the cold water bottles. Hold the card in place as you turn the bottle upside down and rest it on top of one of the warm water bottles. The bottles should be positioned so that they are mouth to mouth, with the card in between. Carefully slip the card out from in between the two bottles. Make sure that you are holding onto the top bottle when you remove the card. What happens to the colored liquids in the two bottles? Hot over cold: This time, place the bottle of warm water on top of the cold water. What happens this time?

So that's how it works… 

Hot air rises because warm air is lighter than cold air. Similarly, warm water is less dense than cold water. When the cold water bottle rests on top of the warm water, the less dense warm water rises to the top bottle and the cold water sinks causing all the mixing you see happening, turning all of the water green. However, when the bottle of warm water is placed on top of the cold water, the more dense cold water stays in the bottom bottle and the less dense warm water is confined to the top bottle. The movement of warm and cold water inside the bottles is referred to as the convection current. Convection currents in the atmosphere are responsible for the formation of thunderstorms as the warm and cold air masses collide. Many different weather phenomena occur because of convection.

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