I whipped up a cover for my baby hot water bottle with some leftover Noro yarn. The little devil is in there for life, there is no means of escape. I'm tempted to felt it now, complete with bottle inside. There is not much room for manoeuvre. Would be interesting to see if the felting is powerful enough to crumple the rubber of the hot water bottle. Suspect this kind of felting may be the stuff of Superman comics but I'll fill the bottle first, just in case.