Instruments are fun!
Homemade ones more so, and ones that look and sound as if a violin and a bass were thrown off the empire state building, landed in a tuba and run over by a taxi and a humpback whale* are the most fun.

So, lets make one!

This is more of an instructional instructable then specific instructions on how to build my instrument.

*I adopted a whale, though if he runs over tuba's is uncertain

Step 1: Materials

What you need (for a stringed instrument, which is what we will be making):

Strings (Golly!) Rubber bands work fine.
Resonant chambers. They amplify the sound. Technology known as "Altoids Tins" work well.
Materials. Whatever you're going to build the structure from. I used (gasp) Knex, because they were in a bucket between my feet at the time of writing.

Step 2: Dont Plan!

Sit around and don't plan your instrument.

Step 3: How Many Strings?

Randomly decide (no planning!) how many strings you will have. For this, I'm choosing 4- to match my cello.

Step 4: Build Frame

This bit is pretty easy. Simply create a frame to stretch your rubber-band strings taut enough to be played. Don't worry about pitch, we'll fix that when we string the bands.

Step 5: Add Extra's

I HATE changing positions when playing cello, so I've made all my instruments so far with movable bridges. You slide them up and down the frame to change position, instead of sliding your hand. So I added that.

Step 6: String Your Strings!

OK, here's The slightly fun part. Pick out however many rubber bands as you have strings, and put them together. Take one, and attach it to the lowest part of your instrument- which part holds the low string, then twirl it around until its twisted up into a single strand. Attach to the other end of the instrument. Repeat with the other strings, but before attaching wrap it around the end a few times to make it higher. E.G. the first string isn't wrapped at all, the second wrap two or three, the third four or six, etc. Just look at the pictures if you're confused.

Step 7: Attach Sounding Chamber(s)

Take your Altoids Tin or whatever you're using, and find where the vibrations are the most, normally where the strings are attached. Rubber band, tape, or glue the chamber to this spot. Feel free to use more than one, on my second I had two. It gave it somewhat of an echo sound.

Step 8: Play! With Video!

Pluck your strings to make sure they are all nice sounding, wrap or unwrap to tune. Then you're done!

Strum with a guitar pick, your fingers, a Knex piece, or whatever! Set it to wacky notes and try to play songs, or pound on the Altoids tins as drums!

Here's a video of three different types in action.