Ice cream, like any solid, melts into a liquid by gaining thermal energy. Thermal energy can be transferred between objects when a hotter object is in contact with a colder object. In this case, the ice cream would be the colder object and the container (which is contact with the air in the room) is hotter.

So, the flow of thermal energy from the air to the container to the ice cream is dictated by a property called thermal conductivity.

Containers made from materials with low thermal conductivity will slow the melting of the ice cream, since thermal energy will not be able to transfer through the container quickly.