High up in the north, there sits a
rock one thousand miles wide by one thousand miles high by one thousand miles
deep.
Once, every thousand years, a little
bird comes to this rock to
sharpen its beak upon.
When the rock has been thus worn away, then but a single day of
eternity will have passed.
Caveat: I may have heard this from some other source, so apologies to that source if this is the case.