We encounter these works, so wisely conceived, at every turn. We shall consider just one particular example here, and that is the flight of migratory birds. To ensure the necessary flying capacity, the bird must be of as light a build as possible.
Excess weight is to be avoided at all costs.
Likewise, use of fuel has to be as efficient as possible. The first step is choosing the most economical cruising speed. Should the bird fly too slowly, it would consume too much fuel simply to stay airborne. If it flies too quickly, it wastes too much energy in overcoming air resistance. Thus we see that there is a definite minimum for the consumption of fuel.