I remember in the mid-60's you could get these little soft plastic PT-boats in your cereal box. The deck was removable, and the idea was to fill the hull with baking soda, then squeeze the deck back on. There was a small hole at the back of the boat, and the baking soda would react with the water, produce bubbles, and propell the PT-boat around the bathtub, pond, etc....

These were a prized item at the time. They were quite small, about an inch & 1/2 long at the most, I'd say.

- LH