An interesting article by Inge H. A. Pettersen (Stavanger, Norway) called “Speed of Sound in Gases Using an Ultrasonic Motion Detector” appears in the May 2002 issue of The Physics Teacher. It provides a good lesson in how motion detectors work, and on how the speed of sound varies. The motion detector is placed pointed upward inside a large bucket with a lid. Normally it reads the distance to the lid, as you would expect. But what happens if you vary the temperature of the air inside the bucket, or inject CO₂ (from dry ice) into the bucket? An original CBL was used for the experiments described, but they can also be done with a LabPro or CBL 2.