Photo: Steven Goodwin [Flickr]
Every kid knows that if you place a ruler over the edge of a desk and flick it, it'll "twang" - and by changing the amount of the overhang, you'll change the pitch.
Steven Goodwin of Blue Dust blog turned this basic idea into a fun (and cheap!) science project you can do with your kids: make a "Rule Organ" out of a set of rulers:
We start by determining the length of each overhang. By knowing how much is needed for the lowest note, C in our case, we can calculate the others mathematically. Finding the lowest note is done in typical school kid fashion by experimenting, unless you have a keyboard, guitar, or other musical instrument and an extra pair of hands, er, to hand.
You will notice that some lengths do not produce notes at all. Very short distances just produce a click, while very long ones make no sound at all. To make a complete octave, the overhang of the lowest note will need to be twice as long as the shortest (highest note), so if the rules you're using only make sounds between 5cm and 8cm you won't get a full octave.
TIP: Hold the rule to the desk as tightly as possible to produce the best audio fidelity (read: twang) possible.