Maachal ben Drusai, for water

Hi,

One must cook food to the stage of "maachal Ben Drusai" before Shabbat, in order to be allowed to leave it on a flame into Shabbat. "Maachal ben Drusai" is the stage of minimum edibility, generally gauged by halachic authorities as 1/3 or 1/2 of "full cooking". At this point, there is reduced concern that one might turn up the flame on Shabbat to speed up the food's cooking.

Regarding heating water, "maachal Ben Drusai" of 1/3 is when water reaches "yad soledet bo"; 1/2 is when it is slightly hotter. Full cooking is when it is bubbling hard.

(Rav Moshe Feinstein, Igrot Moshe Orach Chaim 4:74:Bishul 24)

Have a great day,
Mordechai