The Hot Chocolate Effect

Next time you make yourself a nice cup of hot chocolate, try tapping the side of your mug with your spoon. Then try stirring the hot cocoa so its swirling in the mug, and try tapping the side again. You'll notice the pitch sounds different now!

This is because of something called the "Hot Chocolate Effect".

http://www.health.com/food/how-to-make-healthy-hot-cocoa

The Hot Chocolate Effect is pretty simple, actually. The reason the pitch would change is because of the shape of the container it's in, and the speed of sound traveling through liquid inside. Since the mug's shape isn't changing, then the pitch must be changing because of the speed of sound through the hot cocoa!

But why is the speed of sound through the hot chocolate changing? I thought you would never ask. It's because of the bubbles!

http://www.kraftrecipes.com/recipes/beverages/hot-chocolate.aspx

See when you mix the cocoa powder and milk (or water) together, tiny bubbles form on the surface as you stir. The more you stir, the more bubbles get stirred through all of the cocoa. Since sound travels slower through air than through liquid, like cocoa, the bubbles slow down the speed the sound can travel through the hot chocolate. That's why the pitch sounds lower when the cocoa is moving, and as the cocoa stops swirling around, the bubbles rise back up to the top, making it easier for the sound to travel through the cocoa, and making the pitch higher!