Wind instruments are known to play out of tune when the air temperature is well above or below room temperature, as may occur during outdoor performances. Because the speed of sound in air increases with rising temperature, brass and woodwind instruments tend to play sharp when the air is hot and flat when the air is cold. A simple demonstration of this effect is shown using PVC pipes. Temperature differences sufficient to make the effect audible are easily obtained using ice water and hot tap water. The demonstration is safe, inexpensive, and easy for students to perform. Extensions to the basic procedure are described, including the use of greater temperature extremes and comparison of measurements with theory. The demonstration is suitable for students of many different levels, using either a qualitative or quantitative approach.