This problem nearly ripped the Meyer household apart tonight:
Which glass contains more of its original soda?
Justify your answer.
Download the video.
2011 Mar 04: Updated to add the goods.
2011 Mar 13: 71 comments as of today means we've struck a nerve. Many commenters have put their mark down with an algebraic proof. More interesting to me are those who have included devices for illustrating the proof to their students. That's harder stuff. See MPG's comment:
Consider a similar problem using discrete objects (e.g., playing cards. Take 10 red cards and 10 black cards face down in separate piles. Take four at random from red pile; mix into black pile. Shuffle. Return four random cards face down to red pile. Ask: more black in the red pile or red in the black pile. Try this several times. If you’re not convinced, do it with the faces showing. Apply principle to soda problem.