SALT LAKE CITY (ABC 4 News) - The explosion Wednesday at the Silver Eagle Refinery in Woods Cross was so powerful that it surprised federal investigators on scene and shocked neighbors living nearby. One resident remembers, "There were flames about 40 feet in the air here and it was a great concern."

But the flames didn't damage up to 20 homes in the subdivision close to the refinery. Instead, the damage was caused by a shock wave. This invisible, concussive force radiated from the blast.

Adam Beehler of the University of Utah Physics and Astronomy Department demonstrated how the shock wave works. He used a homemade drum with a rubber top, a smoke making machine and a stack of plastic cups.  Beehler first filled the drum with smoke and then, pointing the open bottom towards the camera, he gave the drum a tap. The smoke rolled out of the bottom in a perfect smoke ring. Then to show that even the drum's small shock wave had force, he pointed the base of the drum at the stack of cups and gave it another tap.  This time the smoke ring rolled through the cups knocking them over.

Of course, the energy released by the refinery blast was many times the power of the wave created by the drum, but the concept is the same.