The Hot Chocolate Effect

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Stir hot chocolate powder into milk or water, and you can recreate this bizarre acoustic phenomenon. Once the powder is mixed in, tapping the side of the cup creates a low pitch that steadily rises as you continue tapping. This is known as the hot chocolate, or allossonic, effect. When you stir, it creates tiny bubbles in the fluid, which changes the effective speed of sound. As the bubbles pop, the speed of sound goes up and the pitch of your tapping gets higher! Stirring the cup up again (even without adding more powder) should lower the pitch once more. (Video credit: C. Kalelkar)

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