on principle im against jokes that confirm the librerrian stereotype
but this one isnt too bad:

Weeding Jokes - for the Librarians
I was weeding juvenile non-fiction books the other day and Harry sent
me the following top ten list in response to my description of the
process. I thought this was pretty cute coming from a non-librarian.
Here it is:

Checklist on J Non-Fic books that can be disposed of (with my
apologies to Letterman's Top 10 Lists):
10) If any part of the text says that "Someday man may go to space."
9) If an illustration of a computer takes more than one page, due to
the size of the machine.
8) If the encyclopedia entry says, "This year's Grammy winners, The
Beatles..."
7) If it mentions the Soviet Union in the present or future tense.
6) If it has no Dewey Decimal number because it predates Dewey.
5) If the word Microsoft refers to something small and not hard.
4) If the book was written before you were born because, really,
nothing important happened before you.
3) If the book refers to New York as New Amsterdam.
2) If you cannot find any mention of World War 2 in an American
history book.

and the #1 reason to get rid of a J Non-Fic book:

1) If a history book only has 2 people listed: Adam and Eve.

source:
http://kinkylibrarian.blogspot.com/2005/04/weeding-jokes-for-librarians.html