Did you ever notice that humidity and "bad hair days" often go hand-in-hand? When humidity increases, the length of hair strands also increases. (Typically, straight hair goes limp and curly hair frizzes!) Scientists in 1783 used that concept to create a cool tool — a "hair hygrometer" — for measuring changes in humidity. Here's how your students can make one:

1. From the piece of oak tag or thin plastic, cut a triangular pointer about 6 inches long. Cut two slits at the bottom of the pointer about 1 inch from the left edge. Tape or hot glue a dime onto the triangle about 2 inches from the pointer's left edge.



2. Cut two slits on the cardboard's top edge about 1/4 inch apart and 1 inch from the left side.



3. Attach the pointer to the cardboard with a pushpin. Place it about 1/2 inch from the left edge and about 3/4 of the way down the side.



4. Attach the hair strand by sliding it through the 2 slits at the top of the cardboard and those at the bottom of the triangle. Use tape or hot glue in both sets of slits to keep the hair in place.



5. Push the pin through the pointer hole so the hair is slightly stretched when the pointer is horizontal. Wiggle the pointer up and down to make sure it can move freely. (The hair should hang vertically and the pointer should be horizontal.)



6. The pointer on your hygrometer is now set to show changes in humidity. When there is a lot of moisture in the air (high humidity), the hair gets just a little bit longer. That makes the pointer droop lower. When the air gets drier (low humidity), the hair gets a little bit shorter and the pointer goes higher.