Or bain marie as it’s often called by fancy foodies. I’ve written about this before, but it’s such an interesting subject I just can’t help myself. A water bath is the classic way to prevent custards from curdling (or “breaking”) because it keeps the temperature of the cooking custard at a constant low. How so? For one because liquid water — unless it’s at a rollicking boil — doesn’t get above 212 degrees Fahrenheit. Beyond that it becomes steam. Thus everything below the water line will remain below boiling, usually below the curdling point (which is around 190 for most still custards) even though the temperature of the oven is 350 or so.
But then if the oven is that hot, what keeps all the water from just turning to steam? The answer is that some of it does turn to steam, however as you may recall from high school physics classes, evaporation is a cooling action. As water molecules leave the surface of the water and depart for the air, the liquid water that remains behind is left cooler for the effort. Neat. Thus the best water baths pans are very broad and shallow, since more surface area means more evaporation and more cooling.