How is the energy put into these instruments so that they make sounds? For guitars and 'ukuleles, that is an easy question. You pluck them. The same goes for the harp. The harpist plucks the string.

The other strings of the orchestra, such as the violin, make sound when they are bowed. How does this work? The bow is made of wood, and the part that comes in contact with the string of a violin is horsehair, from the tails of horses. Under a microscope you can see scales on the hair shaft. It is very rough. When the bow is rubbed against the strings, friction causes the strings to vibrate, and sound is produced. Rosin, the sap of pine trees, is rubbed onto the hairs of the bow to cause them to have even more friction.

String musicians also pluck the strings sometimes. This is called pizzicato.

A piano is a stringed instrument, but isn't bowed. When you press on a key, the mechanism inside causes a rod with a oval-shaped, felt-covered end to strike the strings. This is called the hammer. The hammer strikes three strings for each note on the piano. If you keep the key depressed, the strings will keep vibrating and produce sound. When you release the key, a mechanism dampens or stops the vibration.

The harpsichord is like a piano. It came before the piano. When you press on a key, a mechanism like the one in the piano is activated. But instead of a hammer, the end of the rod has a quill, called the plectrum. This plucks the string! So a harpsichord is more like an 'ukulele than a piano.

If you have a "uke," get it out and let's do some experiments.

• Tune up your instrument. The notes are G, C, E and A. How can you change the pitch of a string? You tighten or loosen the strings. What does this do? It puts more or less force on the string. Physicists call this tension. The more tension on the string, the higher it sounds. The less tension, the lower it sounds.

• Now use a yardstick or meter stick and find the halfway point between the place where the string crosses the neck to the point where it is attached to the body. Press your finger at this place on the fingerboard and pluck either half of the string. How does it sound compared to the open string? Is it twice as high in pitch? It should be. This is called an octave. Now try holding down the string one-fourth of the length from the top. Pluck it and listen!

• There is a third way to change the pitch. The stings on an instrument can have different thicknesses or densities. This is especially noticeable on a piano.

To explore this, find two rubber bands — a thin one and a big, fat one that are the same length. Cut them and tie them together carefully so that you have the same lengths on each side of the knots. Put one knotted end over a tennis ball container, or a yogurt container. Hold the container so the knot is in the middle of the side. Hook the other end of the tied rubber bands over your first finger, with the knot in the crook of the first joint. Pull the tied rubber bands as tight as you can and pluck each.