Well correct me if I'm wrong b/c I don't remember where I read this, but I was under the impression that the magic eraser is made of melamine, a plastic-like material, and it functions like very, very fine sandpaper. I have been using these a lot because I thought they did not have any harsh chemicals in them.
I use the heavy-duty store brand, mostly to clean my shower tile. The heavy duty ones cost twice as much but it is worth it because they last more than one cleaning on the grout.

The Mr. Clean website had some useful information: Magic Eraser

thanks swishina good site!

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married 8 years to a crazy Brit, mommy to a banana, she is 4 and a pineapple who is 1 year, mommy to 3 ~angel~ babies

not sure what's in them but I like the Mr Clean ones bc they have a variety of them - I like the ones with orange layer (some kind of detergent)