Cat House

[Porgie Tirebiter]

Water will run off a truly flat plate for the same reason it runs off the tennis ball. It's attracted to the Barycenter.

Water will remain in a dished or rimmed plate for the exact same reason. It can't flow over the rim, as long as the plane of the flat bottom of the plate is perpendicular to a line from the plate to the Barycenter. Tip the plate so that the bottom is out of perpendicular with that line, and the water will seek the Barycenter.

Fun with Physics and Geometry!