Hot water freezes faster than cold — Mpemba Effect.
Why it is true
Discovered by a Tanzanian student in the 1960s, the phenomenon is still not fully explained by science.
Source: Royal Society
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.