Cuba

In Havana, a physics demonstration had to be called off because sand would not behave as it should.

A physicist was going to show his students how a sandpile grows through small avalanches. He took a handful of sand from behind his institute and let it run in a thin stream through a funnel.

But no avalanches came. Instead, a narrow river of sand formed, running from the top to the edge and travelling around the pile, turn after turn. He could not explain it, and the demonstration had to be called off.

When a colleague in Houston asked him to show the effect, he fetched sand from the university car park. Nothing happened. Only a few months later could he send a videotape from Havana as proof.

A student then tried around 120 kinds of sand from all over the world. Only nine formed the river. The Cuban sand is almost pure quartz, and its grains look entirely ordinary. After many runs it can even stop working, as if something on the surface of the grains wears away.