Take a long human hair, the longer the better, (I designed this in the late '60s when we all had long hair) and tie it to a drinking straw as shown. The pivot should have been placed so that the hair is kept under slight tension.
Tie the other end of the hair to another pin stuck into the notice board.
Place the contraption in your kitchen. The hair will lengthen and the straw "indicator" will move when you boil a kettle and the humidity rises.
Images and article featured on this page were obtained from Bizarre Stuff.