Flashbacks;

Penny shakes her head and says "Oh no, no, you see, in the winter, that seat is close enough to the radiator so that he's warm, yet not so close that he sweats. In the summer, it's directly in the path of a cross-breeze created by opening windows there and there. It faces the television at an angle that isn't direct, so he can still talk to everybody, yet not so wide that the picture looks distorted."

The room is stunned into silence as everyone looks at Penny.

Sheldon gives her a condescending little nod " Perhaps there's hope for you after all."

she turns and smiles up at him.

Sheldon paces up and down the aisle of the store, studying the boxes in his hands.

Penny trails after him, exhausted and frustrated by his fussing. "Why is this such a big deal?"

" Penny, if I'm going to get Leonard a gift, I'm going to do it right. I refuse to let him experience the same childhood trauma I did.
"Ok, I know I'm gonna regret this, but what trauma?"
" On my 12th birthday, I really wanted a titanium centrifuge so, you know, I could separate radioactive isotopes."
Penny shakes her head wearily "Of course, yeah."
" Instead of a titanium centrifuge, my parents bought me- Wow, this is hard."He swallows nervously and presses on " They got me a motorized dirt bike." He looks disgusted
" No!"
" What 12-year-old boy wants a motorized dirt bike?"
"All of them."

" Oh "

"Well, all of them except you. "