The other ingredients are salt and water. Optionally, you can put in some sugar or honey and some vegetable oil or softened butter. I put in a good teaspoonful of salt. I put in a tablespoon of sugar this time, and about a tablespoon of sunflower oil. The flour mixture absorbed nearly 400 milliliters of warm water to form a soft, not-too-sticky dough. It rose very nicely for about 2 hours, in a big bowl, and more than doubled in volume.
The goal with pain de mie is a compact loaf of bread that doesn't have much of a crust. French bakers used to make and sell pain de mie as a standard item, I think, but supermarket loaves have taken over the market. I've tried several brands, but haven't been thrilled with any of them. Some are too sweet. Some have a gummy mouth-feel. Some are just bland. I haven't sliced yesterday's loaf yet, so I don't know what the "crumb" or mie — the soft white inside of the bread — looks like. I'll be slicing the loaf this morning and making croque-monsieur sandwiches for lunch. That's the French version of a grilled ham-and-cheese sandwich. More about that later.