Who defines what is possible?
The smart alec is surely responding "gravity" or something similar.
While that is true, we as humans didn't always KNOW we could get to the moon or space...So take that!
Do you even know what's possible until you hit a wall? I don't actually know how many pull-ups I can do until I go at it.....and realize two is the number (right now). I don't know what I'm possible of writing until I do it.
For fun, if you were in charge of defining what's possible vs impossible, what would you make possible?