water at its surface has different properties than it does below the surface.
the surface acts almost like a "skin", with more adhesion, so that a glass can be filled
above the top rim, with a bead showing.
i saw a 7th grade girl demonstrate the power of this effect on dave letterman this week.
she filled a glass carefully until the water stood up above the rim of the glass. then she gebtly covered the whole glass with a 3x5 index card...and inverted the glass!
nothing spilled; the surface tension held the card
up onto the overfilled glass.
The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed. http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html 
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