Last week I used ice to keep milk fresh but that was not an option when we spent 3 days away from electricity. This time we used a bucket of water. All we were doing was increasing the mass of liquid that would have to be heated. The water and the milk have to be in thermal equilibrium - ie they will settle at the same temperature. Thermal energy from the air will thus be spread through far more molecules of water than if we'd just left the milk. Each molecule gets a smaller share of the thermal energy so the internal energy of each molecule only goes up by a small amount. Temperature is a measure of the internal energy.
