What kinds of things are safe for my mice as chew toys? I put in old toilet paper rolls, and small card board boxes. But I don't think that helps much with grinding down teeth. Can I put old scraps of pine 2X4's in for them? Or is there something in them that could be toxic? Any other suggestions for hard chews. I know there are small chew blocks that can be bought at the pet store. I am looking more for things that I can use that I might otherwise throw away.

Thanks!!