Hot water freezes faster than cold water, and nobody knows why.

This effect has been commented on by Aristotle, Descartes, Francis Bacon, and is now named after Tanzanian high school student Erasto Mpemba in 1963. Boiling water first will also make it freeze faster, but this advantage is undone if then stir the water after boiling.

Explanations vary as to the nature of the Mpemba effect