"In 2011, without warning, a skyscraper in South Korea began to shake uncontrollably and was immediately evacuated. Was it an earthquake? A terrorist attack? No one seemed quite sure. The actual cause emerged later: Twenty-three middle-aged Koreans were having a Tae Bo fitness class in the office gym on the twelfth floor. Their beats had inadvertently matched the building's natural frequency, and this coincidence caused the building to shake at an alarming rate for ten minutes. Frequency is all around us, but really isn't understood." --