Despite a long-standing myth to the contrary, cold water does not boil faster than hot. But hot water may carry more impurities—perhaps the myth arose out of a desire to encourage people to cook with cold water.

Or, perhaps from this odd but true fact (touted by Aristotle): hot water can freeze faster than cold. While I am not a physicist, the over-simplification goes something like this: hot water evaporates faster than cold, and as it loses mass, there is less that has to be frozen.