When we first learn about audio we learn about the basics of frequency, usually by being shown some 2 dimensional graphic of different frequencies. We rarely see a real-life example with the except of perhaps an oscilloscope looking at a series of sign waves.
Here's a great example of the vibrations of acoustic guitar strings looking up from the soundhole. It's pretty cool to look at, but it also shows you frequency in action.
Thanks to my buddy Jesse Jaye for the heads up on this.