This musical noisemaker is based on a Brazilian instrument called a cuica (kwee-kah). The cup acts as an amplifier for the sound made by the string's vibration -- and be prepared, it's surprisingly loud!
Make a hole in the bottom of a paper cup. Poke one end of a 2- to 3-foot length of yarn or thick string through the hole from the inside of the cup. Tie this end to a paper clip, as shown.
To play the cuica, dampen the string with water, then lightly pinch the yarn and drag your fingers down the yarn's length. For a laughing sound, drag your fingers down the yarn in bursts, pausing a split second every few inches.