Re: why does water buldge over the top of a cup without spilling over?

Hello, there!

You have discovered the effect we call "surface tension".  

Water is made up of molecules, and it turns out that water molecules tend to 
grab onto each other (the big-words term for this is "hydrogen bonding"). 
So, those molecules near the surface are grabbed at by those right inside 
the surface and not allowed to flow away when the water bulges over the edge 
of the cup a little bit. There's a limit to how strongly they can hold on, 
though, so eventually the water will spill if you put enough in.

You can see other effects of surface tension when you see the insects called 
water striders that seemingly walk across the surface of ponds. They are 
actually supporting themselves on the held-together water at the surface.

Here's a fun experiment to try. Only tiny amounts of certain other things 
(the big-word term is "surfactant") can cause the water molecules to let go 
from each other and make the bulge vanish. Fill a cup (have it in the sink) 
until the water bulges over the edge. Now wash your hands and leave them 
soapy and let one drop of soapy water drip from your fingers onto the 
surface of the water in the cup.  What happens?