This actually happened at Harvard University in October last
year. In a biology class, the professor was discussing the high
glucose levels found in semen. A female freshman raised her
hand and asked, "If I understand, you're saying there is a lot
of glucose, as in sugar, in semen?"
"That's correct," responded the professor, going on to add
statistical info.
Raising her hand again, she asked, "Then why doesn't it taste
sweet?"
After a stunned silence, the whole class burst out laughing.
The poor girl's face turned bright red, and as she realized
exactly what she had inadvertently said (or rather implied),
she picked up her books without a word and walked out of
class, never to return.
However, as she was going out the door, the professor's reply
was classic. Totally straight-faced he answered her question.
"It doesn't taste sweet because the taste buds for sweetness
are on the tip of your tongue and not the back of your throat.
Have a good day."