On his way out of the flat, John almost bumped into Sherlock.

"Sorry," he said quickly. "I'm in a hurry."

"I know," Sherlock replied, barely glancing at him. "No need to rush, it's just flu."

John blinked and paused with one foot out of the door. "How did you know?" he asked. Even though he'd been living with the man for months now, he still couldn't understand how Sherlock did it.

Sherlock gave him a little smile, no doubt feeling pleased that John had asked so he could once more show how brilliant he was. Or how stupid everyone else was.

"It's quite trivial," he began, prompting John to snort. "You have the shopping list in your pocket, but you're wearing one of your best shirts. Ordinarily you wouldn't change your clothes just to go grocery shopping, so something must have happened that changed your plans at the last minute. I heard your phone ring while I was talking with Mrs. Hudson downstairs, so I can infer that the change was prompted by this call."

John nodded, not that Sherlock needed any encouragement to continue. He was talking so quickly that John felt dizzy trying to follow him.

"So who was the caller?" Sherlock continued. "The change of clothes implies that you're trying to impress whoever you're meeting, so it's most likely Sarah. But it's Tuesday afternoon so she wouldn't ask you on a date right now. She must have told you that she needs you at the surgery, where too many people are coming in with symptoms of the seasonal influenza that's been going around."

If John hadn't known Sherlock better, he would have said that he had listened to the phone call, or installed hidden cameras in their flat, or perhaps both. "But how did you know it was because of the flu?" John asked.

Sherlock shrugged. "Only this morning you remarked that the weather is still warm for October, yet you're wearing a scarf," he pointed out. "Something made you decide to take extra precautions for your health."

"You're correct, as always," John said, and Sherlock gave him a smug grin. "Now I've got to go, I told Sarah I'd be at the surgery as soon as I could."

Sherlock gave him a perplexed look. "But you just agreed that this is just a case of flu," he said. "Flu is, in the vast majority of cases, a non-fatal disease."

"Yes," John replied. "I know. I'm a doctor, remember?"

"So there's no need to rush," Sherlock said. He whisked the shopping list from John's pocket and handed it to him with a theatrical flourish. "And you have plenty of time to do the important things first."

"You mean grocery shopping?" John asked, torn between amusement and annoyance. "I know you think that everything is boring if it doesn't include at least two mysterious deaths, but I can't pick and choose my patients."

Firmly ignoring Sherlock's affronted expression, John sidestepped him and walked out of their flat. "I'll stop by the supermarket on my way back."

"You can't possibly ask me to wait until this evening!" Sherlock exclaimed. "We don't have any baking soda and I need it now."

John frowned. "Why do you...?" he began, then thought better of it and decided he didn't really need to know why Sherlock needed baking soda. With some luck, he'd never find out. "If it's so urgent, you can go and buy it. Do the rest of the shopping too while you're there," he said, giving Sherlock the list.

Sherlock regarded it critically. "But I never go grocery shopping," he pointed out, as if John had just asked him to grow a pair of wings and jump out of the window.

"Now is a good time to start," John replied, and walked away before Sherlock could talk him into changing his mind.

John had several doubts on whether letting Sherlock Holmes to go grocery shopping on his own was a good idea, but it wasn't as if he could ditch his job just because the other man was chronically incapable of doing any housework.

He was halfway to the bus stop when he got the first text. He flicked his phone open and grimaced as he noticed the sender's ID.

What's the point of paying a £1 deposit? Trolleys must be worth more than that. SH

He considered ignoring it, but experience had taught him that it would be a very bad move.

It's to make people put the trolleys back instead of leaving them around.

He hit send and wondered why Sherlock, with all his powers of deduction, hadn't been able to figure that out.

It quickly became clear in the next few minutes that it was because Sherlock was very much unlike the usual Somerfield customer. As if that hadn't been obvious enough.

John's phone beeped again as the bus arrived, and then again as he was sitting down.

The mechanism doesn't look very secure. SH

I'm sure there's a way to force it open with a folded piece of cardboard. SH

Please don't try that, John wrote without much hope.

Sherlock's reply was almost immediate.

Been escorted out of the premises by unsympathetic young man. Customer service is appalling. SH

John chose to ignore that message, and also the following one.

I was right about the trolleys, of course. SH

Of course.

By the time John reached the surgery, Sherlock had managed to locate a Tesco, had successfully acquired a trolley without being thrown out of the store and had already sent John several texts complaining about how the store was too noisy, too crowded and too badly managed.

According to his latest report, he was currently in the frozen products aisle.

Why are the frozen vegetables next to the ice creams instead of the soups? This doesn't make any sense. SH

"Is everything okay?" Sarah asked, popping her head in.

John looked up guiltily from his phone's screen. "Yes," he said. "It's just... It's Sherlock," he said shaking his head, and Sarah sighed in sympathy.

"If he needs your help for a case, you know you can go now," she offered. "I can manage."

"No," John replied quickly. "It's nothing urgent, Sherlock can wait until I finish my shift."

Sarah looked relieved at that. "I'll send the next patient in," she said, and John felt even more guilty at the thought of how many times he'd ran off because of Sherlock's crazy requests.

It was a miracle that Sarah hadn't sacked him yet and he wasn't going to give her more troubles just because Sherlock had problems buying potatoes and instant coffee.

He considered switching off his phone altogether, but he had the nagging suspicion that if he stopped replying Sherlock might do something stupid just to get back at him. He didn't really want to get banned from every Tesco in the kingdom.

Why would anyone want 17 different brands of shampoo?, Sherlock texted him.

John didn't know what to reply, didn't even know if there was anything he could reply without making this situation sound any more ridiculous.

He was saved by the arrival of a new patient, and for a few minutes he could pretend he was a normal doctor with a normal life.

The next time he opened his phone, John had a grand total of eight messages waiting for him.

They ranged from Is it really convenient to buy 3 family packs of breakfast cereals? to Everyone here is stupid! and are all signed SH, as if John knew anyone else who'd ask him which kind of flour would be better to use in a home-made bomb.

Do you mean as a prank or a real bomb?, John replied, and then felt stupid for even asking.

I don't concern myself with pranks. SH

Obviously it would be the most dangerous alternative, otherwise Sherlock would get bored.

"Do I have to take it before or after meals?" the old lady in front of him asked. John explained it for the third time as he texted his reply to Sherlock, one pill after lunch and one after dinner, and come back if it doesn't go away after a couple of days.

Then he wrote the prescription trying to ignore the flurry of incoming messages through which Sherlock explained how he deduced that a man had just been fired and was going to bomb his former workplace in revenge.

"Don't be ridiculous!" John exclaimed, prompting the lady in front of him to mutter indignantly. He had to apologize profusely for his untimely outburst, and by the time she'd stomped out of the door John had received Sherlock's conclusions.

There are fourteen other clues, but those are the most obvious. SH

Will take action. SH

He frowned at the screen but refrained from making any comments.

John was about to diagnose yet another patient with flu when his phone beeped again. He excused himself and opened the message, then blanched as he took in the words.

Come now. SH

He grabbed his jacket and dashed out of the office.

"I've got to run," he called out to Sarah. "Sherlock has caught a bomber!"

Without any further explanations he went straight outside and called a cab. As they rushed back to Baker Street John dialed Sherlock's number several times, but his flatmate didn't pick up.

Typical of him. At any rate, John hoped that Sherlock's hate for phone calls was the only thing stopping him from answering.

There were a couple of police cars parked in front of the supermarket, but no ambulances or fire brigade. John took it as a good sign.

Lestrade himself greeted him with a nod as he stepped out of his cab.

"Where is he?" John asked, and Lestrade jerked a thumb towards the supermarket.

"Still inside," he replied. "You really should explain to him that just because he's got my number he can't call me whenever he wants. I'm not his errand boy."

John shrugged. "I'll try," he said. Both of them knew it would be pointless. "So you caught the bomber?"

"The would-be bomber, yes," Lestrade said rolling his eyes. "We gave him a slap on the wrist and let him go since he hadn't actually done anything yet. He admitted he was planning to bomb some place, but I doubt he'll try anything now. Sherlock scared the crap out of him."

John could picture the scene. "I feel sorry for the man," he said, and went inside.

Sherlock was pacing up and down at the entrance and thankfully looked unscathed.

"Finally!" he said when he spotted John, motioning for him to hurry up. "What took you so long?"

"I've been as quick as I could," John replied. "But it doesn't matter, doesn't it? Lestrade told me that you caught the man already."

Sherlock waved one hand dismissively. "Yes, yes, that doesn't matter. I need your help for something more important," he said, and lowered his voice to a conspiratorial whisper. "Why is this damned machine talking about unexpected items in the bagging area?"