Potato
Sherlock was standing at the kitchen counter, ploughing his long hands through something soft-looking and pale. John moved closer, slipping his jacket off, his eyes narrowing with the cautious curiosity that always washed over him when Sherlock was in the middle of doing something weird. As he approached, he caught the scent of something familiar and had to hold back a grunt of disgust.
"Sherlock, why are you fingering a bowl of mashed potato?"
"It's an experiment," he explained, in a bored tone. His face showed anything but boredom. His mouth was drawn as thin as his plump lips could go, his eyes narrowed in fierce concentration and his pale, high cheeks tinged pink.
"What kind of experiment? Did you get a case?"
Sherlock seemed not to hear him and continued running his hands back and forth through the mashed potato. After a minute of waiting for a response, John gave up and made a cup of tea.
By the time he'd watched a full episode of Doctor Who and finished his tea, Sherlock answered him.
"It's an experiment to see if crushing the potato by hand results in a smoother mash," he admitted. He held out one potato-covered fist to John in what he probably thought was an enticing manner.
"Sherlock, that's disgusting," John cringed, "I hope you washed your hands before you did that."
"Of course I did. I was slicing brains all afternoon. Try it."
"I'm not going to try it, you've been touching it for at least half an hour and if it's not covered in your germs, it's definitely cold by now."
Sherlock started to move towards him and John rose defensively. "I'm not eating it, you insane bastard."
"Come on, I spent hours on this."
"HOURS?! Ugh!"
"Come on, just one bite."
"No."
"Yes- OW!"
Sherlock withdrew his hand from John's mouth and cradled his bitten finger like a child.
"You know what," John snapped, "this potato is disgustingly cold and smooth."
"Smooth?"
"Smooth. Beautifully so. Just use a fork next time you decide to make dinner."