Once upon a time, the animal kingdom decided it must do something to help humanity face the challenges of the world. So the animals organized a school. They adopted a systematic curriculum consisting of running, climbing, swimming, and flying. All animals were required to take all the subjects.
The duck was excellent in swimming, made above average grades in flying, but was extremely poor at running. Since he was slow in running, he had to drop swimming and stay after school for additional practice. This gave the duck’s poor webbed feet callouses, so much so, that he became only average at swimming.