Zack Johnson is an on-and-off boyfriend of Penny's who first appeared in "The Lunar Excitation" (S03E23). He and Penny once got married on a whim while in Las Vegas; Both Zack and Penny were unaware it was a real wedding and ended up having it annulled later. Though usually a friendly, outgoing person, Zack is extremely dimwitted and displays even less understanding of sarcasm than Sheldon does. Despite his stupidity, he is quite interested in science.