Sheldon couldn't sleep. He'd gone to bed at the usual time, but for some reason that his amazingly skilled mind couldn't fathom, he'd been wide awake for hours; tossing and turning.

Like every Friday, today they'd had Chinese takeout for dinner from General Tso's Chicken. Sheldon had had his usual meal of beef with broccoli, while Leonard had vegetable lo mein. Penny had also stayed for dinner, dining on shrimp with lobster sauce.

So, unless General Tso's was using a different recipe to cook their beef, it couldn't be dinner that was keeping Sheldon awake. But whatever was keeping him up, he needed it to stop. Being the theoretical physicist that he was, Sheldon always needed his brain to be running at it maximum potential, even on the weekends. And that meant sleep.

Eventually, he decided that he needed something to calm him down, and decided on a glass of milk. Getting out of bed, he exited his room and made his way into the main area of the apartment he shared with Leonard.

Holding back a yawn, he got a glass out of the cupboard and opened fridge. But when he picked up the carton of milk, something made him pause.

The carton felt lighter than it had when he'd gotten it out at breakfast time. But how could that be? Sheldon was the only occupant of the household that drank the milk and he hadn't used it since that morning. Penny hadn't had any tonight during dinner and Wolowitz and Koothrappali hadn't been over today at all.

"LEONARD!" Sheldon screamed, completely losing what little regard he had for other humans and their wellbeing. This of course dragged his roommate out into the kitchen; his hair messy and his glasses askew.

"Sheldon, it's three in the morning," Leonard said. "What on Earth are you doing up?"

"I can't sleep," Sheldon replied. "So I decided that a glass of milk might help me sleep better, but look," he held out the carton to Leonard and the experimental physicist took it from him. "The milk carton is lighter than it was this morning. You didn't have any, did you?"

"Sheldon, I'm lactose intolerant," Leonard said. "I wouldn't have drunk the milk."

"Then who did?" Sheldon asked. "Because this carton definitely feels lighter."

Leonard sighed and shoved the carton back into his roommate's hand. "You're crazy," he said. "It's all in your head. You're tired and it's making you imagine things."

"I'm not crazy," Sheldon protested. "My mother had me tested."

"Whatever," Leonard mumbled. "I'm going to bed." And with that, he headed back down the corridor, leaving Sheldon standing alone in the kitchen feeling befuddled.

'What could have caused the milk carton to be lighter?' he wondered to himself.

Earlier that day…

Unlike her nerdy friends and roommates, being a waitress and aspiring actress meant that Penny didn't work a nine-to-five job. So when she arrived home at one thirty, after her daily shift at The Cheesecake Factory had finished, all she wanted to do was have a nice hot shower and snuggle up on the couch with a cup of coffee and watch bad reality TV.

However, when she arrived in the kitchen after her shower, dressed in her pyjamas and fluffy pink bathrobe, she realised that she couldn't make herself a coffee. She had run out of milk.

She stood and sulked for a minute before an idea popped into her head. She would walk across the hall and grab some milk from the guy's fridge. 'They won't notice after all,' she thought, quickly locating the emergency key Leonard and Sheldon had given her and racing over to 4A.

Once she had closed the door behind her she put the key in the pocket of her bathrobe, before heading for the fridge and retrieving the carton of milk inside. She quickly placed her coffee cup on the countertop and poured the milk in, before putting in back inside the fridge.

Leonard and Sheldon wouldn't be home for a while yet, so Penny decided it was safe if she drank her coffee in their apartment. For a second, she almost considered sitting in Sheldon's spot, but decided against it and took the wooden chair instead.

However, halfway through her cup of coffee, Penny realised she desperately needed to go to the toilet, which meant she needed to get back to her apartment. While she was confident that she could escape using the milk undetected, she knew Sheldon would notice if she used the bathroom here.

Placing her half-filled coffee cup on the table, she quickly raced out the door and back to her apartment. But after emerging from the bathroom, Penny found the sight of her bed all too inviting. Forgetting all about her coffee, she climbed into her bed and was soon fast asleep.

She awoke a few hours later, when the sunlight streamed through her window and hit her right in the face.

"Uggh," she groaned, sitting up and stretching. "I need coffee."

But upon arriving in the kitchen and realising that her usual coffee cup wasn't in the dishwasher, sink or drying rack, the events from earlier in the day caught up with her. She reached into her pocket and checked the emergency key was still there. To her relief it was, and she quickly ran back over to the apartment across the hall, snatched the cup from where she'd left it on the coffee table and raced out the door.

Penny knew she needed to be quick. She didn't know exactly what time it was, so for all she knew, Sheldon and Leonard could be walking up the stairs right now. She was in such a hurry to get out in case they did come back now and find her; that she didn't hear the emergency key fly out of her pocket and under the bookshelf by the door.

She didn't realised she left the key in Leonard and Sheldon's apartment until she was getting dressed before going over there to have dinner with them later that night, but by the time she left she'd forgotten about it all over again.

Then, the following week, Howard, Raj and Leonard took a trip to Las Vegas. It didn't surprise her that Sheldon had opted to stay behind. But when he was locked out of his 'fortress of solitude' she remembered that she had left the emergency key back in 4A and had to admit to Sheldon that she'd used some milk.

"You're the milk thief!" Sheldon said, outraged. "Leonard said I was crazy, but I knew that carton felt lighter!"