The pitch of your voice is determined, to a large extent, by the time it takes for sound to move up and down inside various bits of your throat. It might help to think of a very simplified model of your throat as an empty beer bottle.
If you blow over the top of the beer bottle, you can make a sound, and the pitch of that sound is set by how long it takes for a vibration of air at the top to run down to the bottom and come back up to the top. If you shorten the distance, say, by putting some water in the bottle, effectively raising the bottom, then the time will be shorter and the pitch will be higher. (Shorter times mean more trips per second, which means higher frequency which you hear as a higher pitch).
Another way to change the time scale involved is to change the gas that's present. Sound travels nearly three time faster in helium than in air, and this is what makes the pitch of your voice go up when you breathe helium.
An amusing experiment you can do at home is to take two identical empty bottles and put one in the freezer for a few hours. Cold air is denser than warmer air. If you blow over the tops of the two bottles you'll find quite a noticeable difference in pitch, that disappears as the cold bottle warms up.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.
