I work with kiddos that are very young, so when our reduce, reuse, recycle lesson comes up, I teach parents and children about regrowth of plants. I've done it with bok choy, celery, and scallions. My understanding is that the scallions (I could be wrong, but its my understanding) won't become onions, but that you can grow onions if you cut the root end of an onion and allow it to root in a glass of water for a few days before planting it in the ground. Plus, you get the scallions!

Beautiful pictures, by the way!