Eruv Tavshilin is one of those halachik Jewish Law solutions that make the impossible possible.
Normally, it is forbidden to prepare food on Yom Tov for use the next day. However, when Yom Tov falls out on Friday, one may prepare for Shabbat on Yom Tov if he symbolically began the preparations before Yom Tov. This symbolic preparation is known as an eiruv tavshilin.Every household must set aside one baked and one cooked item; usually an egg and a matzah/challah are used. The blessing of "al mitzvat eiruv" is recited and the items are held in the hand while saying the prescribed formula. (aish.com)Without preparing food set aside for the final Shabbat meal and making the blessing it's forbidden to cook, boil water or even light Holiday or Shabbat candles.