The 100th episode of the series
Leonard spontaneously decides to ask Penny out on a date. The date is going well, until Leonard pushes the idea of whether they're going to get back together. Penny says that the reasons their relationship never works out is that Leonard always over-thinks everything.
After the date ends (badly), Penny calls him over in the middle of the night for some late night ... entertainment. They decide to not think of their relationship as a relationship, but as new software that they're developing in secret. They continue to argue with each other while carrying on the relationship in secret at night.
It turns out that this whole relationship is only a daydream ... but then Leonard decides to actually ask her out, no doubt setting the stage for their plotlines for the next several episodes, if not the whole rest of season 5.
This Episode's Science
Ectothermy: Ectothermy is the condition of an animal being cold-blooded, and comes up at the beginning of the episode when Sheldon is considering how human life would be different if we had evolved from prehistoric lizards instead of mammals. (Not necessarily dinosaurs, though, since dinosaurs may have been warm-blooded.)
Laser Defense System: Leonard makes a joke about working on a government grant to use high-powered lasers to shoot down incoming ballistic missiles. This is similar to the legendary "star wars" program that Ronald Reagan's administration was working on, largely based on the enthusiastic support of theoretical physicist Edward Teller.
Leonard: Feel like trying something new for dinner? Maybe Indian? Tex-Mex?
Sheldon: Have you ever wondered how humans would be different if they'd evolved from lizards instead of mammals?
Leonard: Okay, let's talk about that.
Sheldon: As you know, lizards, cold-blooded animals, lack the ability to sense temperature, but they do move more sluggishly when it's cold, so lizard weathermen would say things like 'Bring a sweater, it's slow outside.' I love my mind.
Leonard: We all do. Now, how about dinner?
Sheldon: Oh, I assume we'd enjoy insects or smaller lizards. We could also pull each other's tails off and grill them. They'd just grow back.
Leonard: [pretending that he and Penny are getting to know each other on a first date] I'm an experimental physicist at CalTech. Most of my research is with high-powered lasers. Oh, I just got a big government grant to see if they can be used to knock out incoming ballistic missiles.
Penny: Wow! Can they?
Leonard: Oh, God, no. The money's pretty good. And I used the equipment to make my own bat signal.
Penny: [laughing] Bat signal? What are you, some kind of nerd?
Leonard: Not some kind of nerd. I am the king of nerds.
Penny: What does that mean?
Leonard: It means that if anyone displeases me, I don't help them set up their printer.