Skydivers put themselves in a life-or-death situation and
only by careful preparation do they avoid killing themselves. They
do it not only because diving is exhilarating, but also because it
is the closest humans get to a feeling of human-powered flight.
Thinking about free fall led Einstein to his general theory
of relativity. A man in free fall would feel no force of gravity,
and, given no force, would fall at a constant speed; however, on
the contrary gravity causes him to accelerate.
See also in The book of science:
Readings on wikipedia: