In a biology class, the prof was discussing the high glucose levels found 
in semen. A young female (freshman) raised her hand and asked "If I 
understand, you're saying there is a lot of glucose, as in sugar, in male 
semen?"
"That's correct", responded the prof, 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 realized exactly what she had 
inadvertently said (or rather implied), she picked up her books
without a word and walked out of class...and never returned. However, as 
she was going out the door, the Profs reply was classic... Totally 
straight-faced he answered her question, he stated
"It doesn't taste sweet, because the taste-buds for sweetness are on the 
tip of your tongue".