Take today's air, with today's water in it, and COOL it. The sponge shrinks around the same water, RH climbs, and at some exact temperature it hits 100 %. That temperature is the dew point, $T_d$ — say it T-dee). Cool past it and the extra vapour has nowhere to live: it condenses as dew on grass, fog in the valley, or breath you can suddenly see.

The exact machine is the Magnus formula — a logarithm inside a fraction, and the machine will do that part; it takes only the temperature and the humidity. Your job is to AIM it. The forecaster's working rule: every 5 points of humidity below 100 puts the dew point about 1 °C below the air temperature. At 20 °C and 50 % RH: fifty points short, so about 10 °C below — a dew point near 10. The Magnus answer lands within a degree of the rule, and no wrong option ever will. Aim, then pick.