Sheldon and Kripke are forced to work together on a proposal for a new fusion reactor at the university. After exchanging their research, Sheldon is shocked to learn that Kripke's work is far ahead of his own. Kripke attributes this shoddy work to the fact that Sheldon has a girlfriend and Sheldon, anxious to save face, makes up details of their relationship to match Kripke's assumptions. This leads to an intriguing conversation with Penny and Leonard, in which Sheldon admits that he could potentially conceive of a situation where his relationship with Amy might be heading toward containing a physical component.
Meanwhile, Howard and Raj order action figures in their own likenesses. When these figures turn out to be of poor quality, they instead buy a $5,000 3D printer and create their own action figures. However, when Bernadette realizes how Howard is spending their money, she gets furious and closes up the purse strings.
This Episode's Science
Nuclear Fusion - Kripke & Sheldon are working together on a grant proposal for a new fusion reactor for the university. A fusion reactor uses energy to fuse atomic nuclei together, which is the process of nuclear fusion that causes stars to burn. (In stars, this fusion takes place because of intense gravity pulling the atomic nuclei together.) This is not to be confused with a fusion power plant, which would produce more energy than it consumes. Physicists and engineers are still trying to design a fusion power plant that attains this goal, as a major step toward clean energy.
3D Printer - Howard and Raj buy a 3D printer, which allows them to create 3D models of themselves as computer animations and then to actually print these models as physical objects.