Poaching, simmering and boiling are at heart identical cooking methods that in practice yield a variety of results. All that distinguishes one from the other is that each marks a different temperature stage in the process of bringing water to a boil.

As a pot of water heats, the water molecules near the bottom of the pot warm up first. As they get hotter, they move faster. Eventually, they move fast enough to rise toward the surface of the water, where they displace the cooler molecules up above, forcing them to fall to the bottom of the pot, where they in turn are heated and begin to rise.

Ultimately, the water gets hot enough to vaporize the molecules at the bottom of the pot into steam. Because steam is less dense than water, it rises, causing the surface of the water to swirl. This slight movement on the surface of the water is the visual sign that the water is ready for poaching (between 170 and 180F).

When enough steam forms, bubbles at the edge of the pot begin to break through the surface into a simmer (180 to 190F). When bubbles undulate across the entire surface of the water, it is said to be at a gentle boil (210F). And in just 2 more degrees, the surface of the water will break turbulently, cascading with large vigorous bubbles. This stage is called a rolling boil.

It is possible to stop the progression to a rolling boil by reducing the heat as the visual signs for poaching or simmering approach. You know you've got the burner just right when the water maintains the signs of any stage for a full minute. When you add food, the water temperature will drop, but don't turn up the heat. The water should return to the proper temperature within a minute. If it does not, you may have overcrowded the pot with too much food.