A professor had been lecturing on the doppler effect to a really, really dead class. Finally, in sheer exasperation, he pointed to the equations on the board, to be more precise, at the (speed.of.sound-speed.of.object) in the denominator and said:

"This particular portion of the equation shows where the sonic boom comes from. As the speed of the object approaches the speed of sound in air, this part slowly goes to zero. Finally, when the object hits the speed of sound, a division by zero error occurs, reality rips, and all of the air in the surrounding area pours out into the 4th dimension."

The class took notes....