Boil up is commonly used to describe the act of heating water until it reaches its boiling point. However, there are several synonyms that can be used to replace this phrase. These include simmer, stew, bubble, seethe, and brew. Simmer is used when you want to heat a liquid to just below its boiling point. Stew, on the other hand, is used to describe the act of cooking food in a liquid for an extended period of time. Bubble is used when a liquid is heated enough to create bubbles on its surface, while seethe is used when something is boiling violently. Brew, on the other hand, is a synonym for boil up that is used specifically for making tea or coffee.