It was Penny who realised that she and Sheldon had become 'best friends'.
They were sitting next to each other on her couch, laptops out and gamer lingo flying as he helped her with a quest. Penny had gotten over her addiction to Age of Conan with Sheldon's aid long ago, but she still played – but only on Tuesday evenings and only with the physicist by her side, ready to help in case she lapsed back into her old habits (he was also great against goblins). Sheldon had been staring intently at his screen for awhile now as he made his way to a new, extremely magical treasure, when Penny's cry of frustration and shock made him flick his gaze to her laptop. She was being attacked by a group of orcs and without thinking; he abandoned his quest and was soon by her side, slashing at the enemy. When the orcs were all dead, and Queen Penelope had made her way safely through the forest trail (thanks to her guide, Sheldor) Penny paused her game, and turned to look at Sheldon, who had done the same.
"You abandoned your quest to come to my aid – why?" she asked, and he quirked an eyebrow.
"Is that not what friends do? Help one another?"
Penny smiled and found herself giving him an awkward, one armed hug.
"It's what best friends do" she said, and Sheldon paused, looking at the girl who had one arm around his shoulders.
"Am I to understand that our relationship paradigm has progressed to the stage of 'Best Friends?" he asked, and Penny rolled her eyes at his typical way of phrasing 'since when have we been BFFs?'
"I would say so. You're the only one of my friends that knows most of my secrets, and I imagine I know all of yours…"
"Considering I cannot keep one" he added, and she grinned.
"Well, not from me anyway. But yes. I'd say that you're probably my best friend."
"Oh."
"Is that a bad thing?"
He shook his head hurriedly, and gave her a smile before lapsing back into his worried expression.
"Sheldon, what's wrong? Don't you want to be my best friend?"
"Of course I want to be your best friend Penny – It's just…" he paused and she gave him a questioning look.
"I'm going to have to inform Leonard that he's out."