A simple long, thin strip of plastic has a series of ridges along one side. Pull the strip through your thumb and index finger, with your finger nail edge running over the tops of the ridges. You hear a funny hum that sounds like human speech.
Sound is caused by something vibrating in the air. The ridges in the plastic strip make the strip vibrate when rubbed, making a humming sound. If the ridges have the proper depth and spacing, the humming noise can be made to mimic the sound of human speech.