In a biology class, the prof was discussing the high glucose levels 
found in semen.

A young female raised her hand and asked  "If I understand you 
correctly, you're saying there is a lot of glucose, as in sugar, in 
male semen?" 

"That's correct", responded the prof.

Raising her hand again, the girl asked,  "Then why doesn't it taste 
sweet?" 

After a stunned silence, the whole class burst out laughing, and the 
girl's face turned bright red as she realized exactly what she had 
said. She picked up her books and walked out of class.

As she was going out the door, the Prof, always the educator, replied 
with a straight face "It doesn't taste sweet, because the taste-buds 
for sweetness are on the tip of your tongue."