Get a transparent basin and fill it with warm water.

Now place some ice cubes in the basin.

Quickly add a few drops of food colouring to the water on one side of the basin.

Now watch what happens and draw a sketch of what you see.

(In this case we have caused convection by adding a cold material at the top. It produces the same effect as adding a hot one at the bottom.)

You can try this advanced test:

Place a thermal box in the basin – the kind you freeze and then put in food bags to keep the contents cool. It will sink to the bottom of the basin. What happens now?