Ok, for small (very small) bodies to flip over the threshhold, but the nature of going from one state to the next involves a change of volume, especially at the boiling point, and means that new material supplants the original, and the process starts all over again. This is Convection heating (cooling – same process only in reverse). Otherwise we would have an instant change for all of the material, when the very last molecule has gained (lost, if cooling) sufficient heat to effect the change. Imagine a kettle boiling then! Or the condensation of water vapor if it all happened at once?

Even explosive material does have a very fast, but finite reaction time, though this is chemical I realise, it is still reminiscent of the mechanical nature of the processes we observe. The Internal Combustion Engine and the Steam Engine are good examples, as is Dynamite for the purely chemical example. That they react almost instantly, is credit to various physical properties, chosen judiciously, but they can be un-even in practice, though the dynamics of the system can mask the actual event. See Formula 1 and Space rockets – they weren’t fully developed overnight.

I suggest there is a hysteresis effect happening.

As a oneliner though, I agree in general with the comment.

What do others think?

Ivan.