Why Can't We Eat Silica Gel?

Most of us are aware, by adulthood, that silica gel is used as a desiccant, or a substance to keep the area around it dry. That's why you see them packed in with all kinds of new purchases, like shoes, pills, snack foods, and electronics. You don't want your stuff arriving moldy or rusted after it's been in a warehouse for a while. These little packets of silica gel beads always say "DO NOT EAT" as if they are poison. 

But what is silica gel, anyway? You may be surprised to learn that it has properties that make it useful in other applications besides absorbing humidity. A miracle material, indeed. And what's worst that could happen if someone did eat it? After all, the warning looks a bit like a challenge to people who don't like to be ordered around.