Sheldon: Now, knowing Penny, the obvious answer is, they engaged in coitus. But, since that's what it looked like, we can rule that out. Let's put on our thinking cap, shall we? (Mimes doing so) Raj is from India, a tropical country. Third world hygiene. Parasitic infections are common, such as pinworms. The procedure for diagnosing pinworms is to wait until the subject is asleep, and the worms crawl out of the rectum for air. (Leonard spits out his food) Yes, just like that. Penny could have been inspecting Raj's anal region for parasites. Oh, boy. That's a true blue friend.
Leonard: They slept together, Sherlock.
Sheldon: No, you weren't listening. She said, it's not what it looks like.
Leonard: She lied.
Sheldon: Oh. Well don't I look silly sitting here wearing this?