In a biology class at Harvard University, 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, the girl 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 realised 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, not the back of your throat. Have a good day."