system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If water starts evaporating before 100 degrees Centrigrade, what happens when it reaches 100?

Does it all evaporate? How is water at 99 degrees different from water at 100 degrees?
gpt
Remember, heating isn't uniform through out a liquid and the bit you're sampling the temperature of won't be representative of the entire lot.

However! Yes, if you have a volume of water and somehow uniformly make it all hit 100C, it would all become vapour.

At 99C technically none of the water will evaporate with no external forces acting on it. But due to the flux of energy between molecules, this 99C is more an average and at any one time a molecule will "transfer energy" to another temporarily (making a molecule 98C and another 100C), giving it enough energy to change phase to a gas form. Put very simply, this is why warm water (eg 40C) will evaporate faster than cool water (10C); due to this random moving of energy between molecules and the likelihood of one having high enough energy to evaporate. 

This is of course well before other factors (relative humidity, layering, pressure etc.) get taken into account.