as i remember them, they acted like if you compressed some foam, and then held it in compressed form with some water solluable glue.
BUT -
the ones i had were not foam - they were, IIRC, some sort of jelly stuff. you put this small pill or something in a bowl of water, and it expanded into some jelly scorpion or something (which then, if you tried to lift it out, fell apart)
id imagine they were made of some industrial effluent in the 80s. probably made of foam now.