As a kid with two older brothers, I knew how important it was to be the first to crack open a new box of cereal and sift through the contents for the toy inside. I remember fishing out a plastic toy submarine and I absolutely loved it. When you filled the submarine’s compartment with baking powder and dropped it in a container of water, the submarine would—seemingly magically—dive and surface. When I came across Bob Knetzger’s SpudMarine project in MAKE Volume 26, these fond memories flooded back to me.
While the heyday of cereal box toys may be over, you can still make your own diving submarine toy out of a potato. It can take some trial and error, but once you find the perfect ratio of wood and tuber, you can pack it with baking powder (not baking soda) and watch it go.