Hot water can freeze faster than cold water.

This phenomenon has been observed for centuries but was not properly introduced to the scientific community until 1969. Since that time, it has commonly been referred to as the Mpemba effect, named after the Nigerian high school student who rediscovered this back in the ‘60s.

Mpemba first noticed this back in 1963 when making ice cream for a school project. The students mixed boiling milk with sugar and were told to let the milk cool before refrigerating it. Impatient, Mpemba disregarded directions and put his milk in right away without letting it cool. Inexplicably, his milk cooled before the rest of the class! But his physics teacher said this was not possible.

He later found that local ice cream vendors did the same thing to speed up the freezing process! But Mpemba was still criticized at school for his “bogus” science, even though he had hard evidence with which he could back his theory up. A physics professor later visited the high school and heard what Mpemba had to say. He replicated the process and couldn’t explain the phenomenon either, so he and Mpemba published their results in 1969.

For a more in-depth explanation.