How come heating milk in the microwave is safe?
When metal is exposed to microwave radiation, an electric potential difference can develop as the microwaves generate electric charge in parts of the metal. Flowing electricity can cause sparks as electrons migrate to places of lower potential. Solid metal is susceptible to this because its electrons are relatively loose, making it a good conductor of electricity.

Calcium (Ca) in milk is bonded with other atoms (mostly as calcium phosphate), so it doesn't behave as a solid metal would: first because its electrons are secured in bonds with other atoms, and second, because the Ca atoms are not aggregated together but intermixed with nonconducting ones.