A couple of idle musings but I hope interesting ones: 1. If a drop of water lands in a still pond, circular concentric waves spread out. Is it possible to reverse this process with a large circular wave generator such that the waves converge and meet in the middle, and a drop of liquid jumps out of the pool? Has anyone ever demonstrated this? 2. A guitar string makes a definite pitch because it's constrained by the nut and fret, allowing only waves that divide the string length a whole number of times. Could you make a fixed-pitch musical instrument from circles of metal (similar to the triangle but closed and, obviously, circular)? Would the oscillations of the metal be of a too-high frequency for this to work properly? The inspiration for the latter is the electron in a hydrogen atom.