Why does the temperature of a substance remain constant during its melting point or boiling point ?

Why does the temperature remain constant during the melting of ice even though heat is supplied continuously ?

Why does the temperature of a liquid become constant at its boiling point all thorough it is still being heated ?

Why does the temperature remain constant during the boiling of water even though heat is supplied continuously ?

When water boils, why does its temperature remain constant?

For any substance, Why does the temperature remain constant during the change of state?