edge then center
Here is a simple experiment that I've seen described but I
have never seen an explanation that seemed to make sense.
Fill a glass about half full with water. Put a couple of bits
of paper, cork or anything that will float onto the water.
The bits will eventually move to the edges of the water. Fill
the glass to overflowing. The water should bulge up so that it
is above the rim of the glass. The bits will then move to the
center of the water's surface. Why?
This will also work with larger objects such as a cork or
a bottle cap.