Overhead transparency* (You can use one that's been printed on that you no longer need, as I did)
Small, shallow dish (or a jar lid works)
Cut the transparency in half. Stack the two pieces on top of each other, roll them into a tube and paperclip at each end.
Place some water in the shallow dish. Add several drops of food color (don't use yellow for this demonstration, you need something darker to show up).
Stand the tube you made in the dish of liquid and watch what happens. You may want to roll up a piece of plain white paper and slip it inside your tube to improve visibility.
As you watch, you'll see the colored water creep up the tube. You're seeing evidence of adhesion - water's desire to stick to things other than itself. So much so that it overcomes gravity to keep working its way up the tube.
Didn't photograph real well, but the color did make it all the way up the tube.
*Does my knowledge and use of transparencies make me old? I feel like all the new teachers out there are laughing at me and my out-dated ways. That no one uses an overhead any more, they all project things using their computers and SmartBoards. Regardless, don't get rid of your overhead projectors, there are cool science demonstrations you can do with an overhead that you can't do with your fancy computers! :)