Elmo, Rosita, and Rudy are making a submarine! They use cardboard boxes to build their submarine but it's not big enough to fit all three of them. This is a problem! They wonder if there's something big enough that's not too heavy. What if they combine two boxes to make one big box? Let's try! It works. They didn't give up and added on windows, propellers, and rudders to their submarine and pretended to explore the ocean using their imagination.