This is a popular MYTH about how stone tools were made by the ancient aboriginals of North America.  
Heating flint or chert and then exposing it to any amount of cold water will cause it to
EXPLODE, CRACK,
OR POT LID.  
This myth has it's routes, believe it or not, from Indians who gave the story to the first white
explorers as sort of a "Joke's on you".  After those first wise jokers passed the false information on, it's been
said that younger generations heard this and thought it was the truth.  It only takes a generation or two for
any people group to lose knowledge of a particular skill if there is no need to practice it or use it.  That was
the case with the Indians.  The introduction of steel points and knives through trade, made flintknapping an
unnecessary process that, for the most part, slipped out of use in the late 1800's or so.  This myth was also
popularized when Edgar Burroughs wrote the book "Tarzan".  In this fictional novel, Burroughs described
Tarzan heating a stone blade and dropping cold water on it to remove flakes from it.

   Needless to say, those how have tried to replicate this myth have been seriously injured.  I can say with
100% confidence that stone tools were not made with this technique and that it should never be tried.  If
seeing modern flintknapped tools and points and comparing them to ones of history isn't enough to make you
sure, there is a safe experiment you can do in the comfort of your own kitchen.  Take several ice cubes out
of your freezer and put them in a dry room temperature glass.  Now take cold tap water and fill the glass.  
The resulting popping, snapping, and cracking that you see and hear are the results of a severe temperature
change that has been abruptly applied to the ice cubes in the glass.  In the case of the ice cubes, the solid
cubes are very cold and the water is considerably warmer when applied to them.  I hope you can see the
similiarities to the heated stone and cold water, as the results are much the same, just minus the explosion,
911 call, and blindness.
Answer