Explain why a glass of water quickly warms up when placed on a hot surface but does not cool as quickly when placed on a cold surface. Assume same temperature differences between the water and the surfaces in both the cases.

On a hot surface the bottom layer of water is heated, becomes less dense and rises, while cooler water sinks to take its place. These convection currents carry heat quickly through the whole glass. On a cold surface the bottom layer is cooled, becomes denser and simply stays at the bottom, so no convection current is set up. Heat then moves through the water only by conduction, and since water is a poor conductor of heat, the glass cools much more slowly.