Heres something interesting I noticed the other day. I had just made a cup of coffee in a ceramic mug, and had just finished stirring it. Absent mindedly, I started picking up my spoon and dropping it into the cup at roughly half second intervals. On doing this, I noticed something very peculiar that baffled me at first. Each time the spoon hit the bottom of the mug, it rang with a frequency that was noticably higher than the time just before. I kept this up for a while, experimenting by dropping the spoon from different heights, I tried increasing and decresing the time intervals between spoon drops, but none of these things changed the fact that the frequency increased steadily each time the spoon dropped. After a while, I noticed that the coffee in the mug was still swirling, but very slowly. I stirred it rigorously, and this time the frequency was low and once again increased steadily as the swirling slowed. Does anyone have any ideas as to why this happens? Here was my initial thought: The swirling coffee exerts a centrifugal force on the walls of the mug, as the swirling slows due to friction, this force decreases. Could it be that this centrifugal force exerts enough strain of the walls of the mug to alter the pitch of the sound produced to the extent, that the change in frequency can be clearly heard after just half a seconds change to the centrifugal force. The reason Im asking is that as Im sure you can imagine, this force must be miniscule, and even when the coffee was hardly swirling at all, the change in frequency was still easilly discernable. Im curious to hear any other possible explanations you people might be able to think up.