Originally, the idea was for Penny to move into 4A. It made sense – she spent more time there than she did in her own apartment, after all. It was logical, economical, and the least disruptive of options.
So quite how they come to be moving into a slightly shabby little Craftsman bungalow near the University is never quite clear. But with Sheldon's increased professorial salary, and Penny's semi-regular tv role, it isn't a financial stretch, and Penny still gets a secret kick out of signing 'Mrs Penny Cooper' on things. They've been there a week, when Penny finds herself running out of the kitchen in response to a muffled yell from Sheldon.
She bursts in through the door of his study, expecting anything from a papercut to a flame-war about forks, to find Sheldon engaged in a staring contest with a small black kitten. Well, he's glaring at it. It merely blinks lazily back, then marches unevenly across his keyboard, and uses him a stepping stone to the ground.
"It sprang in through the window like a veritable panther, Penny." Sheldon is rather belatedly pulling the sash down. "It's the vanguard of a feline horde, I just know it."
"It's one kitten, Sheldon. A tiny, harmless kitten."
Penny crouches down, and rubs the furry little head butting at her ankles. It winds around her hand, purring like a small motor, then heads past her for the living room.
"Penny, stop it, stop it...oh, good lord."
The kitten has jumped up onto the end of the couch, paddles lazily at the striped cushion.
"It's in my spot."
Sheldon glares. In the way of cats everywhere, the kitten promptly flings up a leg and washes it's bottom at him. She thinks that he's going to stroke out right there. His jaw sets with purpose.
"We are not keeping it, Penny."
Due to what Sheldon refers to as a 'satanic disposition', and a bad habit of sitting in doorways and letting the heat out, they call it Maxwell.