the story of an ice-cream

There are 2 reasons for the phenomnon. At first the surface of the ice-cream is colder than its surroundings and thus heat is transferred from its surroundings to its surface. This is the zeroth law of thermodynamics. Since water vapor is nearby, water vapor will transfer its heat to the surface of the ice-cream.

The second part is latent heat. When matters change phrases such as from vapor to liquid or from liquid to solid, latent heat is released to the surroundings. When water vapor transfers it heat capacity to the surface of the ice-cream, it turns into liquid form (water).

You can look at it the other way: the surface of the ice-cream absorbs heat from water vapor and thus turns water vapor into liquid. In turns, the surroundings (air or in this case the ice-cream) is being warmed by the condensation.

For more information:
http://en.wikipedia.org/wiki/Latent_heat

2009-03-21 02:50:36 補充：
Sorry, "2 reasons" should read "2 parts".

Ans:
Since the cup of ice-cream is taken away from the freezer, the temperature of the cup is about 0 celcious. When water vapour (formly of gasous state)of the surrounding makes contact with the cup, it condenses (change from gasous state to liquid state) on the cup surface.