Determining the speed of sound in the lab is always a student favorite. Most find it hard to believe that they could possibly calculate the speed of sound accurately using only a tuning fork and water. This simple, yet intriguing demonstration shows just how easy it can be. Partially fill the resonance tube with water. Sound a tuning fork over the open end of the tube. Then adjust the water level until the tube resonates at the same frequency. As the water level changes in the tube, a standing wave produces an antinode at the top of the tube that causes resonance. The speed of sound can then be calculated using the water level and the frequency. Measurement markings on the tube provide accurate results in calculating the classic 1/4 wavelength produced in a closed cylinder. Students can also determine the frequency of an unknown tuning fork by using the speed of sound in air and the height of water in the tube.