I would like to know about the theory behind this method to achieve an 100% oxygen saturation standard, According to my manuals:

"For the standard 100% Oxygen saturation, fill a cup with 4 mm of water and put the probe above it, the probe must be dry at all times, this conditions correspond to air saturated with 100% oxygen and water vapor"

anyone knows why is this condition corresponding to 100% saturation?

thank you very much in advance.