Let's talk only about pure water, and only water at or close
to atmospheric pressure.
At the surface between air and water, or between steam and
water, water boils at 100 °C. Water boils at 100 °C if
there is already a bubble of steam (or air) present. But in
the absence of bubbles, water can be heated above
100 °C. There are two reasons. First, to make a
stable bubble, a lot of water molecules in the same small area
must form steam. This is improbable. Second, it takes extra energy
to form the bubble itself: energy to push the water out of the
way, and energy to make the surface between water and steam. Once
a bubble forms (a process called nucleation), it is easy to
increase its size. So the superheated water nearby evaporates very
quickly, producing a large volume of steam.
Smooth containers do not have bubbles of air clinging to their
sides. Rough walled or scratched containers may hold microscopic
bubbles in their cracks. These become nuclei for boiling.
Even a crack that is fully filled with water can be a boiling
nucleus because it reduces the required area of the water-vapour