You've seen the little packets. They're everywhere. They're tossed into boxes and bottles with your consumer electronics and vitamin supplements. "Do not eat," say the packets. And sometimes they also say that they contain silica gel, which is a dessicant.
Silica gel helps prevent rust, corrosion, mildew, odor, spoilage, excessive stickiness, and other types of damage caused by moisture or condensation. It is not actually a gel, but a porous, granular form of silica, which is the same material found in quartz. Developed just before World War I, it was used in Allied gas masks. During World War II it was used to keep penicillin dry. It remains a very popular adsorbent, most recently having found a new career as a type of cat litter.
Silica gel is inert and non-toxic. Why do the packets say "Do not eat"? Probably because it just isn't certified for human consumption. And really, there's no reason you'd want to eat it, except that the packets tell you not to. It probably wouldn't hurt you if you ate it, although you might want to accompany it with a large glass of water since it would probably dry your mouth out. You'll notice that the packets do not say "POISON". The exception to this is blue silica gel, which is colored with cobalt chloride and is toxic.
Silica gel can absorb about 40 percent of its weight in moisture. When it becomes saturated, it can be dried out in a conventional oven at 300 degrees Fahrenheit (150-ish Celsius) and used again and again (although you probably wouldn't want to try it with the kitty litter).
9 January 2004: vruba says I have put silica gel on my tongue to see what it was like, and I can testify that it does in fact dry your mouth out.