A term used in the making of red wines, the cap is the layer of grape skins, pulp, pips (seeds), and other solids that naturally rise to the top of a fermenting vat. The cap acts as a natural barrier against unwanted oxidation. Fermenting generates a lot of unwanted heat, however, so to cool the process down—and to extract beneficial phenolic compounds, which help in developing color, flavor, and tannins, from the solids— the cap must be broken up periodically, either by pumping the must over the cap or punching down the cap into the must.