The hot air in an oven cooks vegetables and fruits relatively slowly, for several reasons. First, air is not as dense a medium as water or oil, so air molecules collide with the food less often, and take longer to impart energy to it. Second, a cool object in a hot oven develops a stagnant “boundary layer” of air molecules and water vapor that slows the collision rate even further. (A convection fan speeds cooking by circulating the air more rapidly and disrupting the boundary layer.) Third, in a dry atmosphere the food’s moisture evaporates from the surface, and this evaporation absorbs most of the incoming energy, only a fraction of which gets to the center. So baking is much less efficient than boiling or frying.