Thingmaker is a product by Mattel started in 1964. The toy came in kits, like Creepy Crawlers, with die cast moulds that you you heat in the open-face hot plate oven to about 390 degrees F (199 degrees C) and add the Plastigoop which would cure , when cooled by the placing in the water tray to form rubbery replicas.
There were even moulds sold separately like Superman and Tarzan. There was different sets for making soldiers, or dragons or Creeple People and more. They even made moulds the would fit together and you would insert the goop into a hole that would fill the cavity.
When cool you would have a 3d character , like the soldiers and moulds shown in my photos. It was a lot of fun making these different things and we were smart enough, back then, not to injure ourselves or burn the house down. Through the years they have added safety precautions to the kits but they still sell them.
To read more about Thingmakers check out Wikipedia.
See the photos from my collection (and Tims!) and some cool Creepy Crawler videos by reading more!