water floats on oil?

[Roger Mitchell]

This is just an observation I have noticed recently when doing some PCR reactions. After setting up the reactions I place the tubes opened into the PCR 
 machine (Perkin Elmer 480) and add two drops of mineral oil (Sigma) by a
 sterile pasteur pipette, close the lids, then immediately begin the relevant
 file for the reactions. Later I have checked the machine to ensure all is
 going well,(you never can tell when FATE strikes you down) and noticed condensation inside the lid of some of the tubes, though not all. 
     After replacing the condensate from the top of the tube to the bottom
 and letting the PCR continue on it's way, I pondered (a new scientific
 pastime leading to madness, dementia and wishing for lottery wins) HOW
 and WHY the water can escape from under the oil layer to be deposited by
 evaporation onto the lid of the tube? ( by the way the PCR reaction does
 work, despite this transgression of the laws of PHYSICS)
  I am using thin-walled tubes from Sorenson, Bioscience INC. (much cheaper
 than `P-E' and just as good) but maybe the reason is the way I add the
 oil by dropping it into the tubes and the water splashes up the sides.
 Although the aqueous reaction should fall back and slide down under the
 oil to be protected from evaporation by the heating of the machine.
  Has anyone else noticed this effect in some of their PCR tubes or have
 a suggestion as to the cause and alleviation of this peculiar effect.
   As I said before this does not affect the PCR reaction but could 
 reduce the yields if the condensation is not returned to it's proper
 place in the tube. Other people in our lab have also noticed this 
 phenomenon and have no explanation. Is this just a peculiarity of the
 laws of physics in our lab or have other people noticed it in their
 longer experience with the PCR reaction.
 Thanking all in anticipation for comments and suggestions
 Roger Mitchell.