What is impossible?

The definition is: "not possible; unable to be, exist, happen, etc."

Given what we know from history, is there anything that we should feel comfortable throwing into this category? 100 yeas ago the list of what was impossible would include walking on the moon. 200 years ago, it would have included driving a car or speaking to someone across the globe by telephone. None of these impossibilities actually were impossible, yet all were universally accepted as impossible at one time.
(http://inspirinet.com/content/impossible-or-possible)

At one time, one thing may be impossible...
at another, we'll have to wait and see...