Step-by-step explanation:
Steaming up or fogging happens when steam condenses on the mirror. Steam emerging from hot water can condense on a colder surface. That’s the reason you can see the result on a mirror instantaneously. Obviously, for a bathroom mirror to steam up, the steam that originates at the shower spray (or the bathtub) has to travel through the cooler air to reach the mirror. Since air tends to heat up easily, the mirror can steam up fast.