Basically, heat from the earth’s interior produces a molten layer of magma. The tectonic plates float on top of this material. The molten magma is undergoing convection, bringing the hottest material to the surface and allowing cooler material to sink. As the hot magma reaches the top, it spreads sideways (nowhere else to go) and any plates floating on it move also.

You can see this in a pot of simmering water containing small pasta, such as orzo. The grains will move out to the edge, sink to the bottom and the be brought back up with the hotter water.

As Mike says, but the most beautiful convection currents that I have ever seen made visible have been in solvents containing very fine aluminium powder. Really lovely. It makes no rafts unfortunately, but you can form some artificially to float on top.

Hmm... custard... now boiling custard may be better yet, because you can eat the continental masses afterwards. One of my favourites...