This is the secret : Sound moves in waves, and when sound waves travel through the air fast enough, they heat up anything they hit.

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A. What good is the siren? Well, when the sound waves from the siren are turned on a pot of water, the water will boil in two minutes. Paper held in front of the siren goes up in flames in a few seconds.

B. Dr. Rudnick's siren pushes the sound waves along so fast that people cannot hear the sound, although dogs, cats, dolphins, and bats may. Sound waves that travel so fast that people cannot hear them are called 'ultrasound.:

C. At first, ultrasonic waves did not seem to be of much use. Scientists knew they were a new source of power but did not put them to work. Then the scientists learned how to point these waves. like a flashlight, at any object or spot.