A full boil and a simmer are not the same: at a full boil, water is at 100°C / 212°F, large bubbles rise continuously from the entire surface, and the liquid visibly churns. At a gentle simmer, the temperature is 85–95°C / 185–205°F, the bubbles are small and sparse, and the surface only trembles in places. A 15°C / 27°F difference alone determines whether soup stays clear, meat becomes tender, and pasta falls apart. Looking at the pot is enough to see the difference.
The difference can be seen by watching the bubbles. At a full boil, bubbles break away from the bottom of the pot, reach the surface, and burst; the movement is so rapid that the ingredients swirl in the water. At a gentle simmer, bubbles mostly rise from the edges, a few per second, while the center of the surface stays almost still. There is also a third state between the two: if no bubbles appear at the surface but steam rises and the water’s surface trembles slightly, the temperature is around 70–80°C / 160–175°F; this is called a trembling simmer. Recipes call for these three states separately, and calling all of them “boiling” is a source of confusion.
A full boil is needed for only a limited number of tasks. Add pasta to plenty of fully boiling water; otherwise, the starch dissolves slowly and the pasta sticks together. Briefly dip green vegetables in fully boiling water to preserve their color. Boil jam and syrup so the water evaporates quickly and they reach the right consistency. Dried legumes should be boiled vigorously for the first 10 minutes, then the heat should be lowered and they should be left to simmer. Foods with shells, such as potatoes and eggs, are less likely to crack in simmering water than in boiling water.
Almost everything else calls for a gentle simmer. Long-cooking meats such as stews and boiled meat become tough in rapidly boiling water because vigorous movement breaks the fibers and makes them bind together. If the same meat is held for hours at around 90°C / 195°F, the connective tissue melts into gelatin and the meat becomes tender enough to fall apart with a fork. Boiling stock or chicken broth makes it cloudy: fat and protein particles disperse in the water and do not separate again. Yogurt-based soups curdle if boiled, and milk-based desserts stick to the bottom of the pot.
Keeping a simmer steady takes more than just turning down the heat. If the lid is fully closed, the temperature inside rises and a simmer turns into a boil; leave the lid ajar or remove it completely. If the pot is larger than the burner, the center may boil while the edges stay still, so choose a pot that fits the burner. The most precise method is a thermometer: if a kitchen thermometer immersed in the liquid reads 85–95°C / 185–205°F, the simmer is properly set. Without a thermometer, the number of bubbles is a useful guide, and you get used to judging it after a few tries; checking the pot now and then works better than trying to find the right heat setting from the start.