So I did.
It does in fact expand. A lot. Like on the walls and door of my microwave, a lot. And it is not easy to clean up, because...
It is neither mold-able nor fluffy so no little hands can enjoy playing with it (Not to mention the fact that it is still soap. Foul tasting, eye burning soap. I kind of didn't think of that when I decided this was a great idea.). It is brittle. You touch it and it disintegrates into soap dust. Teeny, tiny soap dust that sticks to your hand or the surface it is on if you try to brush it into a pile so you can clean it up. Unless...
You try to wipe it up with a damp rag, then it turns into soap paste that again, sticks to everything and is now foaming.
Now, I have a pile of soap dust:
My only option was to convince my three year old it was "snow soap:. It was a pretty easy sell because, well, he's three and still believes what I tell him. So, I ever so gently, picked it up and put it in a Ziploc bag to use at bath time. Worth the effort it took to clean soap expansion overflow off my microwave? No. But it could have been a much less painful process if I knew then what I know now (isn't that true of most things?).
Learn from my experience and if you want to try to make your own "snow soap" cut a bar in half, put it on a disposable plate and microwave it for 1.5-2 minutes. That should solve the overflow problem and you can dump it right off the plate into a baggie keeping the soap dust to a minimum. If you breed little scientist who want to know why the soap "explodes" there is a great explanation here . Happy mess making!