Impossible.

Impossible is defined as "Incapable of having existence or of occurring". The human species has redefined it as a challenge.

There was a time when making thousands of copies of a single book was impossible. That time is gone.

There was a time when the earth was flat and when trying to sail beyond the Earth's edge would lead to an infinite fall. That time is gone.

There was a time when heavier than air machines could never fly. That time is gone.

There was a time when traveling faster than sound was impossible. That time is gone.

There was a time when traveling to the moon and beyond was impossible. That time is gone.

"Impossible" is gone. It only remains as a misdefined word in the dictionary.