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Water molecules have a strong attraction for one another. Inside the
glass, the molecules that are surrounded by other molecules are attracted
in all directions. But the molecules at the surface have no water above
them, so they are strongly attracted downwards by the molecules below
them.
These attractive forces are strong enough to keep the water from spilling
over the top of the glass, even when the level rises quite a bit beyond
it. But eventually the volume of water above the rim of the glass becomes
too great for the surface tension to hold, and the water will spill.
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| | What happens when you add a few drops of dishwashing liquid to the water in the glass? Can you add more pennies than before, or fewer? | |
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