Also, the material in hay that causes the teeth to wear down is called silica. It's the main component of glass, beach sand, and those little packets that come in your shoes. Plants incorporate it into their stems for structure and stability. The silica in fresh grass is called hydrated silica, because the silica molecules are surrounded by water molecules. The silica in dry grass has much less water to interact with, so it is less hydrated. Hydrated silica is more abrasive on teeth than dehydrated silica. This was told to me by the vet at UW-vet school when I first took [my bunny] in (in some different terms, I just translated the chemistry into layperson terms for you).