If the water is in a tall metal can rather than in a flat dish, the paradoxical effect disappears. We argued that temperature gradients in the tall can were short-circuited by heat conduction through its metal walls. The question has certainly made me reluctant to take accepted wisdom for granted when it comes to observations which do not fit preconceived notions of what is correct. Penicuik Midlothian a. The classic experiment uses two metal buckets placed in the open air on a cold, preferably windy, night.
Stationary water is a poor conductor of heat and ice forms on the top and around the sides.
There is no means by which the warmer water can come into contact with the cold bucket and transfer its energy to the outside. Even though the first ice forms later, complete solidification of the hot water can occur more quickly than if the water starts off cold. The conditions are critical. The containers must be large enough to sustain convection with a small temperature gradient, but small enough to extract heat quickly from the bucket's surfaces. Forced air cooling on a windy night helps.