One of my A level students yesterday had never seen convection currents in water, so I set up this simple experiment for him. Into a large beaker I poured some cold water and at the bottom I placed a small coloured crystal.
I heated the water with a burner and watch as the water heated, the water coloured by the crystal rose up in the beaker - not straight as the books suggest but in a more spiral direction until they reached the top when they spread out even more.
The water showed that it was less dense than the water around it because it was hotter and therefore it was lighter and rose to the surface - albeit not in a quite vertical direction.
There we are warm water rises and disperses the heat around the beaker from the bottom to the top.