Answer: Episode title!
If this one fooled you, then you're not alone!
For anyone who's ever taken a biology class, you probably know that protons are teeny particles that can be found inside of an atom's nucleus. It would seem logical, then, that there might be an actual theory about protons that undergo wild transformations. But, as it turns out, the only "Proton Transmogrification" theory out there exists on The Big Bang Theory.
In this episode, Sheldon was forced to cope with the loss of Professor Proton, while Leonard and Penny reflected on all of the changes their relationship has undergone over the years.