Physics Bumper Sticker Goes Over Head [Pic]

It has to do with the doppler effect; When something either moves toward or away from a vantage point, it's frequency of both light and sound change. The sound that comes out of a firetruck`s horn is a constant pitch, but it sounds like the pitch is oscillating because it is rotating and changing it`s distance from you, and thus changing the frequency. This is also why stars that are close in proximity to the Earth appear blue in the sky, and far away stars are red (henze the red-shift phenomenon).