It takes a lot of heat to turn liquid water to gaseous water (steam), again because of the strength of hydrogen bonding. Heat adds energy to the water molecules, giving them more kinetic activity until the molecules are free from all or almost all of the hydrogen bonds, allowing for escape into the gas phase (steam water). The energy required to vaporize liquid water is called the latent heat of vaporization; this is what allows foods to cook at or near 100°C (212°F) as long as there is considerable water content in the tissue.
Simmering large pieces of meat in water or stock allows the cook to keep a constant temperature; the water stays near boiling point because any excess energy (heat) is being spent as water molecules escape the water phase into the vapor phase. Modernist cooks also called molecular gastronomists use a method called sous vide, French for cooking under vacuum. Imagine a fish with a large midsection and long tapering thin end.
The thinner parts of the fish would reach a higher temperature earlier while cooking than would the thicker middle portion of the fish. Sous vide‐style cooking allows for a precise, even temperature maintained throughout the food. This means the food is cooked to the same temperature in the core of the food without excess temperature at the surface of the food.