Repeat until it won’t anymore:
So when the computer stores a file on the hard drive, it puts it anywhere, right?
And then you store another file, so it’s next.
And you add a page to the first, so it won’t fit where it was, so it’s in two pieces, right?
And then you store another file, so it’s next.
And then you delete a file, so there’s a blank area.
So the next file fills the hole but won’t fit there, so it’s in two pieces.
And it’s a mechanical spinning drive, so getting each piece back takes longer.
Any questions?

This computer has been assaulted by a dust storm.
It was locking up from heat in general, and clay particles shorting out the video board.
It made a full recovery, and no longer lives on the floor next to a screen door.

(With apologies to the “got milk?” commercials…)