We have two new puppies ( they are now about 4 and 5 months old. We have a 10' x 10' pen in the yard until they are trained on the invisible fence. Anyway, had a LOT of rain yesterday. The puppies were inside or in the screened porch for most of the day, but not in the pen. Went out this morning and most of it is just plain mushy mud. The rest of the yard is ok. So what can I put down in there that will ease the mud situation, but not kill what's left of the grass? Any ideas?