For the most part I would have to agree with grayson too. But technically it was the electrons orbiting the atoms in the wall that stopped your head from traveling in the direction it was going.[/b]
So the matter of the wall was of little consequence, it was the walls energy/mass that stopped your head from its gloriouse trip.
Many Worlds and an Anti-Universe
Judging by the "many worlds" interpretation, there are an infinite number of universes and each time an event occurs an alternate (yet almost identical) universe is created in which the event didn't take place -
t.