Say you were on a satellite throwing out a rock at random. And say we split up the surface of Earth into 2256 equally large parts. Your rock would have to fall at the exact spot representing the key you would have to find (in the first go). Well, that spot has an area in itself of 4.4×10-63 square meters. Well, that size is too small to make sense of (it has an order of magnitude of 1/1045 square nanometers). A proton alone would cover 1047 of these slices.
There are a bunch of other great examples.