What happens to the temperature of a mug of hot cocoa when cold milk is mixed in​to it?

When cold milk is mixed into a mug of hot cocoa, the temperature of the hot cocoa will decrease. This is because the cold milk absorbs heat from the hot cocoa as it mixes, causing the temperature of the hot cocoa to drop. The amount by which the temperature of the hot cocoa decreases will depend on the initial temperature of the hot cocoa, the amount of cold milk added, and the specific heat capacity of the milk.

It's worth noting that the temperature of the mixture will eventually reach a new equilibrium, at which point the temperature will stop changing. This new equilibrium temperature will be somewhere between the initial temperature of the hot cocoa and the temperature of the cold milk.