found it on another forum
"This happened at Harvard University in October last year. In a biology class, the professor was discussing the high glucose levels found in semen which gives the sperm all the energy for their journey. A female freshman raised her hand and asked, "If I understand you correctly, 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."