I was having an argument with my sister last night about the limits of scientific knowledge (her beliefs encompass a more "spiritual" world view than mine) and she brought up quantum mechanics as an example of how weird things are possible (as peddlers of hocus-pocus love doing). Leaving aside the point that just because QM looks weird to us, doesn't mean the whole world is inherently "weird", and certainly shouldn't be used to try and pass off any mumbo-jumbo idea as worthy of credence... The example she brought up was that of, "if you threw a tennis ball at a wall for long enough, eventually it would pass straight through the wall". This is something I remember being said from my school days, I think to illustrate the principle of quantum tunnelling. Now I know it's dangerous to extrapolate quantum weirdness to the macroscopic world, and my scientific spidey sense is telling me that this is one of those times when a metaphor used to explain quantum weirdness to a layman is taken literally. So, what's the score? Assuming that I threw a tennis ball against a brick wall from now until infinity, and assuming neither ball nor wall were damaged nor deteriorated over time, would it just so happen that one time the ball would pass through the wall? Is it physically possible (and just very, very unlikely), or is it completely physically impossible?