When University College physicist Denis Osborne visited Mkwawa Secondary School in Iringa, Tanzania, in 1963, he little expected the question he got from student Erasto Mpemba:

“If you take two similar containers with equal volumes of water, one at 35°C and the other at 100°C, and put them into a freezer, the one that started at 100°C freezes first. Why?”

The other students derided Mpemba, but he was right — in cooking class he’d noticed that hot ice cream mixes froze more quickly than cold ones.

Osborne confirmed the effect and shared a publication with Mpemba in 1969. What’s behind “the Mpemba effect” is still something of a mystery — it seems to be a combined result of supercooling, convection, evaporation, and the insulating effect of frost. (If you want to conduct your own experiment, start with containers at 35°C and 5°C.)