Boiling (100°C/212°F at sea level) and simmering (85–95°C) are moist-heat methods that transfer heat through water to food. A full boil produces large, rolling bubbles and is used for pasta and blanching; a simmer produces small, gentle bubbles ideal for soups, sauces, and braises that would toughen or break apart at a full boil. Salting pasta water is not merely flavoring — it raises the boiling point slightly and seasons food from the inside.
Practice visually identifying a boil versus a simmer. Cook pasta from raw to al dente and compare to overcooked texture. Make a simple vegetable soup and practice reducing heat to maintain a steady simmer for 20+ minutes.
Water boils at 100°C (212°F) at sea level because that is the temperature at which its vapor pressure equals atmospheric pressure — water molecules are escaping as steam as fast as they are being replaced. Until that threshold is reached, adding more heat just raises the temperature; once you hit 100°C, additional heat converts water to steam rather than raising temperature further. This is why a "rolling boil" is not meaningfully hotter than a "gentle boil" — both are at 100°C. Understanding this from your kitchen safety background helps you recognize that covering a pot (which raises pressure slightly above atmospheric) does speed up boiling slightly, but an uncovered pot's water temperature is effectively capped at 100°C no matter how high the flame.
A simmer sits in the range of 85–95°C — hot enough to cook food through, but below the vigorous agitation threshold of a full boil. The practical difference is enormous. When you boil a delicate protein like fish or egg, the violent tumbling of a full boil tears apart soft tissue and creates a rubbery, overcooked texture. The same heat applied gently at a simmer cooks the food through with much less mechanical disruption. Soups and braises simmered for 30–60 minutes develop flavors as compounds from vegetables, meat, and aromatics dissolve slowly into the liquid — a process that a violent boil would cloud up with emulsified fat and break tender ingredients apart.
Pasta is one of the few foods where a full rolling boil is actually recommended. Pasta needs the turbulent water movement to keep pieces from sticking together and to ensure even hydration of the starch as it swells. The starch granules in pasta begin gelatinizing around 65–70°C, but cooking at a proper boil ensures this happens rapidly and uniformly. The famous instruction to salt pasta water before adding pasta does two things: it raises the boiling point very slightly (less than 1°C, negligible for timing) and, more importantly, it seasons the pasta from the inside as water is absorbed — a step that cannot be replicated by salting the dish afterward.
Blanching — briefly plunging vegetables into boiling water before transferring them to ice water — uses both techniques in sequence. The boiling water rapidly deactivates the enzymes that cause vegetables to turn brown and mushy over time. The ice bath immediately drops the temperature below 4°C, stopping the cooking process and preserving the bright color and crunch. These two steps together are the foundation for many meal-prep workflows: blanched vegetables can be stored safely (connecting directly to food storage principles you'll encounter next) and reheated without turning grey or soft.