If you're going to test marshmallows, you've got to use some Peeps out of principle. Here are some really spooky Halloween ghosts--they shrink by themselves.
We took an empty plastic bottle, drilled a hole in the lid, and stuck in a basketball air pump, wrapping rubber bands around the pump insert to help create a seal. Though the seal wasn't perfect, we were able to pump in enough air to increase the pressure in the bottle, and sink our ghosts.
The eerie moaning isn't ghostly howling, it's air escaping from our leaky bottle.