Someone is moving, and someone is leaving.
Newtons third law teaches us that for every action, there is an equal and opposite reaction.
Its also a convenient way for the writers to get our favorite physicists back on campus working together.

Credit: Monty Brinton/CBS
The idea of being tossed out without a care in the world stings the egos of all three guys.
Why does the government need so many comic books, action figures, and commemorable swords?
When does it end?
How dare you, Uncle Sam.
Within seconds, Leonard realizes that they have walked into an empty apartment on the third floor.
Onward and upward to the fourth floor!
The guys share their terrible news and count on the rest of the group to cheer them up.
The next morning, Amy butters Sheldon up by preparing his favorite breakfast bland oatmeal.
Sheldon accepts the gesture with a humble heart.
He tells Amy that he doesnt know how he ever got by without her.
In fact, hes completely dependent on her and finds great comfort knowing she is there.
I like sweet Sheldon.
Amy persists that Sheldon doesnt need her.
This sends up a red flag.
Sheldon pushes until Amy finally admits that she has been offered a summer research fellowship at Princeton.
Sheldon insists she accept the offer and then hustles to the bathroom to be alone with his colon.
If Amy doesnt go, shell regret it.
Besides, with Amy gone, Howard has a friend he can play with.
It seems that the engineer suffers from anxiety when he doesnt have a project.
He gets a little too clingy and could use someone who likes kite fighting and video game playing.
This is a win/win/win.
His woman done left him and now he has the blues.
Leonard puts the kibosh on this old roommate nonsense and encourages Sheldon to not dwell on the negative.
He should be happy for Amy.
Plus, he has the chance to be a loving and supportive boyfriend.
Amy thanks him with a big smooch.
Sheldon returns the affection by suggesting that they partake in amorous activities, even though its not Amys birthday.
THEORETICAL LAUGHTERBERNADETTE: So what happens next?HOWARD: Its phase two.
BERNADETTE: You okay?
Do you want me to put on some Neil Diamond?HOWARD: No.
Youll get all sexed up, and Im not in the mood.
The salesman said it can survive a plane crash.
Perhaps you’re free to fly inside it.
SHELDON: Ive never lived by myself.
What if I become strange and eccentric?