For flight to occur in Aves, four physical forces (thrust and drag, lift and weight) must be favorably combined. In order for birds to balance these forces, certain physical characteristics are required. Asymmetrical wings, found on all flying birds with the exception of hummingbirds, help in the production of thrust and lift. Anything that moves through the air produces drag due to friction. The aerodynamic body of a bird can reduce drag, but when stopping or slowing down a bird will use its tail and feet to increase drag. Weight is the largest obstacle birds must overcome in order to fly. An animal can more easily attain flight by reducing its absolute weight. Birds evolved from other theropod dinosaurs that had already gone through a phase of size reduction during the Middle Jurassic, combined with rapid evolutionary changes. Flying birds during their evolution further reduced relative weight through several characteristics such as the loss of teeth, shrinkage of the gonads out of mating season, and fusion of bones. Teeth were replaced by a lightweight bill made of keratin, the food being processed by the bird's gizzard. Other advanced physical characteristics evolved for flight are a keel for the attachment of flight muscles and an enlarged cerebellum for fine motor coordination. These were gradual changes, though, and not strict conditions for flight: the first birds had teeth, at best a small keel and relatively unfused bones. Pneumatic bone, that is hollow or filled with air sacs, has often been seen as an adaptation reducing weight, but it was already present in non-flying dinosaurs, and birds on average do not have a lighter skeleton than mammals of the same size. The same is true for the furcula, a bone which enhances skeletal bracing for the stresses of flight.