We've done other surface tension experiments and they never fail to impress. Both my boys loved this and considering how simple it was, it captivated their attention for much longer than I'd anticipated.
Here's how we tied water in knots.
What You Need
an empty 2-liter soda bottle
Trim the top off the bottle, above the label. Peel off the label. Use the nail to poke several holes between 2-5 holes about 1/4-inch apart. Make sure they are not further apart than this.
Conduct the Experiment
Fill the bottle with water quite full. Watch the water stream out the holes. Now run your fingers over the bottle's surface and pinch the water streams together (i.e. tying it in knots). Slide your hands across the holes again to separate them.
How it Works
The cohesive nature of the water molecules (which are attracted together) cause the streams of water to "bond," seemingly tying the water streams in knots.
This great activity came from Brenda Walpole's book 175 Science Experiments to Amuse & Amaze Your Friend.