Now You See It, Now You Don't
Manufacturers of water heaters changed their assembly methods between about 1993 and 1996. About 90 percent of all water heaters manufactured during that period (estimated 21 million units) were built with defective plastic dip tubes. The dip tube is a pipe which carries fresh cold water to the burner at the bottom of the tank, and helps separate the fresh cold water from the hot water which is about to be distributed to the house.
The defective dip tubes sometimes break down inside the water heater and cause plastic chips to flow to water faucets. The chips do not pose a health risk, but they can decrease water flow from household faucets and appliances. The lack of a functional dip tube in the water heater will diminish water heater efficiency and effectiveness (hot water doesn't 'last' very long).
Unfortunately, the warranty on this problem has expired. Normally a defective dip tube can be replaced, but if there are other problems with the water heater it may make more sense to simply replace the whole unit. You should check with a plumber if you think your water heater is having problems.
Visit the web page of the American Water Works Association for more information.