Elmo is playing with his doll, Baby David, in a large washtub. The doll is floating in a tube, and Zoe comes by and asks if Rocco, her pet rock, can swim too. Elmo reluctantly agrees. When Zoe puts Rocco in the water, he immediately sinks to the bottom. Leela explains to her that not all rocks float, some sink. Zoe suggests that if Rocco had a boat he would be able to float. Leela thinks this is a great idea for an experiment! Elmo comes up with the idea of Rocco floating on pencils since he saw sticks float in a lake once. With Leela's help, they experiment to see if the pencils float. They do! But when they place Rocco on the pencils he falls right off. Zoe brings back a piece of paper, which is wider and flatter for him to float on. The paper floats, but once they put Rocco on, it sinks. Next Elmo and Zoe try aluminum foil, since it's flatter and wider than pencils but stronger than paper. The foil on its own floats, but when they put Rocco on top, it sinks too! Elmo comes up with the great idea to build a boat for Rocco. With Leela's help, they become engineers and use what they've learned to design a boat by wrapping the aluminum foil around the pencils. It works! Everyone is excited as they watch Rocco float in the water!
