Here's what you need: One mug of hot coffee in a ceramic mug. One spoon. 1) Swirl coffee with spoon as if you are stirring in sugar or milk for thirty seconds. (In fact, feel free to actually swirl in sugar or milk. It makes no difference.) 2) Start quickly tapping the inside bottom of the mug with the tip of the spoon. 3) Observe the change in pitch of the tapping. Why does the pitch go up? It doesn't matter whether the liquid continues to swirl or not. You can stop the swirl of the coffee with the spoon and the same change in pitch is observed. Why?