If (as US Presidential hopeful Ben Carson believes) the Pyramids in Egypt were built as grain storage silos, how do you suppose they filled and emptied them, and how long would a Pyramid full of grain last?

Is there any indication of exterior openings in the tops or sides?

I’m curious how any thinking person could believe that the Pyramids were gain silos for more than a minute or two, much less 17 years and going.