Check this out Yaw'll

My favourite measuring device is the Yaw String. What is a Yaw String you may ask? Well it would have to be one of the most simplistic measuring devices out there. It is basically, a piece of yarn stuck with sticky tape to the perspex canopy of a glider (other aircraft do also use this device, however it is not as common).
  The yaw string measures the amount of sideslip occurring in flight. Sideslip is when an aircraft is moving forward but facing sideways and is undesirable because it greatly increases drag, thus also affecting the lift of the aircraft and when gliding, lift is your best friend.
There exists a more complicated device which most powered aircraft use called a slip indicator ball, however this is not as responsive as the string and also requires looking down to the control panel to check it. The string allows for constant monitoring of sideslip as it is always in your field of view. I really like this technology because its functionality and simplicity have stood the test of time in an industry where high tech devices are the norm. In fact, according to Wikipedia (sorry Robbie), the yaw string was the first ever flight instrument, used from day dot by the Wright brothers on their 1902 glider.