It turns out that hot water freezes faster than cold water. The speed at which molecules move when at a higher temperature causes the bottom to freeze first (as opposed to forming an icy film on top of the liquid that forms when cold water is used, which in turn insulates the substance underneath). Without the top layer, as the unfrozen water remains exposed, it will freeze faster than if cold water had been used. Try it!
Source: http://math.ucr.edu/home/baez/physics/General/hot_water.html
