How come orange juice has a bitter taste if you drink it shortly after brushing your teeth with minty toothpaste?
The sodium lauryl sulphate (SDS) in toothpaste desensitizes your sweet-sensing tastebuds. That leaves just the sour and bitter ones working. This search provides more info, if you want it.
Speaking of searches. . .
http://boards.straightdope.com/sdmb/showthread.php?threadid=6427
Chances are most questions you can think of have already been asked, either on the SD archives or the SDMB. Doesn’t mean that anyone had a satisfactory answer, but a lot of times some interesting info comes out.