"In my case," said the student to the sex researcher, "when I get
it part way in, my vision blurs.  And when it's all the way in, I
can't see a thing."

"Now, that's an most interesting optical reaction, that may well have
anatomical as well as physiological basis." the researcher replied.
"If you don't mind young man, I should like to have a look at it."

So, the student volunteer shrugged and stuck out his tongue.

(blaaaaaaaaaaaaarrrrrrrrrrrrrrrrpp!)