To comprehend the biomechanics of flight, a few simple physical principles must be kept in mind. Above all, you must recognize that air is a fluid, just like water. It is not a liquid, like water, but is a called a fluid because the force needed to deform it is dependent on how fast it is deformed; not how much it is deformed (try moving your hand quickly, then slowly through a basin of water for an example). Solids are substances for which the force needed to deform the substance is dependent on the extent of deformation rather than the rate of deformation (so it takes the same amount of force to break a pencil quickly as it does to do it slowly; try this with a pencil that is devoid of sentimental value to you). As is common in nature, there are subtle gradations between the artificial dichotomy of fluids and solids; we have given you a generalized definition for each of the two ends of the continuum. We'll use "fluid" interchangeably with "air" here, and "object" interchangeably with "animal".