toothpaste and toothbrush
orange juice
salty food
sweet food
sour food

  First, gather some foods to taste.  I tried orange juice, chocolate, corn chips, pistachio nuts, and sesame crackers.   Then you need to brush your teeth.  I suggest that you only use a little bit of toothpaste, as you are going to do this several times.  To get the best results, you should also brush your tongue, which dentists suggest anyway.  You want to get the entire surface of your tongue covered with the foam.

  As soon as you finish brushing, taste a sip of orange juice.  Keep the juice at the front of your tongue, where you had the most toothpaste.  Notice the taste.  Ick!!!  Not very good, is it?  Brush your teeth again, to remove the taste of the orange juice and get ready for the next test.  This time, taste something sweet.  Again, try to keep the food at the front of your tongue. 

  Where is the sweet flavor?  It will either be missing, or very faint. Try the same thing with something salty, again brushing first.  Again, the salt flavor is weak or missing.  Then try it with something sour.  I used lemon juice.  The sour comes through just fine. 

  What is happening?  Toothpaste contains detergent, to make it foam and help it remove food from your teeth.  This detergent coats your taste buds, which blocks the sweet and salty flavors.  Sour and bitter flavors do not seem to be blocked.  This is why you get the bad taste from the orange juice.  You
are tasting the sour and bitter part, without the sweet part.

This experiment  was re-printed with permission from Robert Krampf