http://www.youtube.com/watch?v=j-zczJXSxnw

above is the link to a youtube video about the Tacoma Narrows Bridge that collapsed in November 1940. Now this bridge did not just collapse. First it started moving like a wave headed for the shore. The reason this happened was because of resonance. Resonance is when 2 things have the same pitch (or they vibrate at the same frequency). When something of that frequency occurs, it causes the other thing to vibrate because it has the same frequency. For example- let's say you have two pianos, tuned exactly to the same pitch. When you strike Middle C on one piano, the middle C string on the other piano is going to vibrate because one has been struck at the same frequency. I find this whole concept very intriguing and suggest anyone and everyone to watch the above video. ENJOY! =)