read this months ago but I can't find it again after several searches.
it's about a dumb girl and her smart childhood friend
it's a humorous single chapter manga where he or they are revising for a test (I think in his room) she has problems (she's........really stupid) so he helps her learn the biology of "reproduction", the ending is something along the lines of getting the test results back and her answers are her saying "babies come from when "his name" puts his dick in me and spurts his stuff" or variants on that
anyone got a clue?

finally found it
the second chapter of IKEGAMI Tatsuya's volume "Mitsuiro no Kousoku Ihan" entitled "I'm stupid"
[url]http://www.mangaupdates.com/series.html?id=64997[/url]
it was a bit darker than I had remembered