Cooking a liquid at a temperature of 212°F (100CC) is called boiling, and liquid is said to be boiling when it is seething, rolling and bubbling.
In most recipes, the liquid is brought to boiling point (or the ingredients are put into boiling liquid), but the heat is then reduced and the cooking is carried out at a lower temperature. This is done because continued boiling may cause the ingredients to become tough, to shrink and to lose their flavour.
Sauces and gravies are often allowed to boil so that the quantity is reduced and the flavour more concentrated. Heavy puddings, such as suet pudding, and green and other vegetables are also boiled.