What to Do: Wet a length of yarn (ours measures about 3 feet) and tie it to the handle of a liquid measuring cup. Fill the cup approximately two-thirds full of water. Position the yarn over the spout, then place the loose end in a drinking glass. Hold the measuring cup about 1 1/2 feet from the glass and use your free hand to pull the yarn taut over the center of the glass, as shown. Beginning slowly (you may spill a little), pour the water along the yarn. When the water begins "sticking," you can pour it more quickly. Make this trick even more drip-defying by using a longer piece of yarn to see how far you can pour with no spills.
Why It Works: Water molecules like to stick together. In physics, this is called cohesion. By soaking the yarn first, you create a liquid surface for the stream of water to cling to.