It is a widely accepted fact in the piano world that changes in temperature can drastically affect the pitch of strings in a piano. However, I've never seen a real demonstration of how quickly pitch can change due to a small temperature shift.
I recently came across this short video by Christopher LaBarre that does an excellent job demonstrating the effect of temperature on string pitch. With only a few quick passes of felt along a piano string, Christopher builds up enough heat in the string to drop the pitch by around 5 Hertz (heating the string causes it to expand, resulting in lower tension relative to the unheated string).
For those of you who are interested in seeing the calculations behind the relationship between temperature and string tension/pitch, take a look at this thread on PhysicsForums.com.
Big thanks to C.J.'s Pianos and Christopher LaBarre for putting together this great video.