Which freezes faster, hot water or cold water? It's obvious, isn't it? Before you make up your mind, watch this:
This effect has been known for centuries and was first described by Aristotle. In modern times it was rediscovered by Erasto Mpemba in 1963. He was a Tanzanian schoolboy at the time and noticed the effect while preparing ice cream in a cookery class.
Here is a layman's explanation.
For you physics braniacs here is a technical paper outlining 7 experiments.
Amazingly, such a seemingly simple (yet counterintuitive) phenomenon still has no conclusive explanation.
I'm wondering if the sun will rise tomorrow morning.