The term “rolling boil” is used to describe an especially vigorous state in a pot of boiling water. You may also hear a rolling boil referred to as a “roiling boil.” Rolling boils are often called for when it comes to blanching vegetables and flash-cooking other foods in boiling water. Foods like pasta, which start cooking at the boil, sometimes also benefit from being dropping into a pot of water at a rolling boil.

Given that all water boils at the same temperature, 212 degrees Fahrenheit (100 degrees Celsius), differentiating between different “types” of boil might seem odd. However, water in fact behaves slightly differently depending on how long it has been at the boil, and whether or not the water is still being heated. When water reaches a rolling boil, it means that the boil is so aggressive that it cannot be disturbed or disrupted by stirring, or by dropping ingredients into the water.

You can tell when water is starting to boil because large bubbles begin to rise up from the bottom of the pot and burst open at the surface of the water. Water at the early stages of the boil can easily be disrupted with a quick stir with a spoon or whisk. When water is at the rolling boil, the bubbles are much larger, and they bubble up so quickly that when the water is stirred, bubbles will continue to form. Clouds of steam will also roll from the top of the water, which can pose a burn hazard if cooks are not careful.

One reason many recipes call for a “rolling boil” is to ensure that water is really at the boil when ingredients are added. Water actually starts to bubble below the boiling stage, and when ingredients are added before water reaches the boil, the water does not get a chance to fully boil. When ingredients are added at the rolling boil, the water is so hot and so agitated that it actually stays at the boil unless the heat is turned down or a high volume of ingredients is added.

Incidentally, if you happen to be involved in the starting with hot vs starting with cold water debate, the reason many people recommend starting with cold water is that many minerals are soluble in hot water. If your pipes are lined with layers of mineral material and hot water runs through them, the heat will dissolve the minerals, and they will wind up in your cooking pot. Mineral deposits can alter the flavor of the food, and, depending on the minerals, they can also be harmful to your health. These deposits can also accumulate in cooking pots. Cold water has significantly lower levels of dissolved minerals.