Phil Kangas expressed precisely :
"Jeff Wisnia"

I'm always interested in learning new things to try and spotted this one in
a book of puzzles SWIMBO just gave me. I just had to try it.

Put a paper or Styrofoam cup with water in it on a kitchen or postal scale
and note its weight. Leave it on the scale. (A glass of water might be too
heavy for the scale.)

Now stick a finger or two into the water without touching the side or
bottom of the cup and watch what happens.

The scale reading increases.

Did you expect that? I didn't.

I'm sure any physicists here will explain why.

Jeff
-- Jeffry Wisnia

Easy. You added the mass of your finger below waterline
to the mass of water already in the cup.

It's not the mass, it's the displacement of a volume of water.

Eureka!!