A ball is floating in a beaker of water in an elevator. What happens to
the ball when the elevator accelerates upward? Good question for your
"bright" students!
Think about this before checking out the video below.
I guess you could demonstrate an elevator accelerating downward -- free
fall -- by dropping a (plastic) beaker above a garbage pail to see what
the ball does while falling. Might be hard to observe this fast motion, so
a video camera would be useful.