If you have ever been to the circus, you have seen a tightrope walker cross an open space by walking on a rope or wire. For this experiment, we will see water do the same thing.

You will need:

- two plastic cups (Use cheap ones that you don't mind putting a hole in.)
- yarn or thick, cotton string about two feet long.
- a nail
- water

Use the nail to carefully poke a hole just below the lip of each plastic cup. Stick one end of the string through the hole, going from the inside towards the outside. Tie a knot in the string on the outside of the cup, so that the end of the string will not pull through the hole. Do the same thing at the other end. It is important that the string that connects the two cups comes from the inside of the two cups.

Now fill one cup with water and wet the entire sting. (This part should be done over the sink or outside, just in case) Hold one cup in each hand. Move them apart until the string is tight. Hold the cup with the water about 6 inches higher than the empty cup. Tip the cup of water so that the water flows slowly at the point where the string goes over the edge. Instead of falling, the water will flow along the string into the other cup. It may take a little practice to get it to flow smoothly, but once you get the hang of it, it should be easy to do.

The water flows along the string because water is naturally "sticky." The water molecule is polar and tends to stick to other water molecules (giving it surface tension.) It also sticks to many other substances, such as the string. Between sticking to the string and to other water molecules, it is easier for the water to flow along the string than it is for it to break loose and fall. Chemists use this idea to avoid spilling liquids when they pour them. If you place a glass rod (or a spoon) against the edge of the container of liquid and place the other end into the container where you want the liquid, it will flow down the rod and not spill onto the table. It works just as well in the kitchen, so the next time your iced tea pitcher seems to dribble more tea on the table that into your glass, pour the tea along a teaspoon and you can avoid the mess.