Prompt was from Kitschgeist: Exactly what it says on the tin.
"That's all it says: exactly what it says on the tin."
Watson frowned in confusion. "Exactly what it says on the tin? And that is all that the tin says?"
"Exactly," Gregson confirmed. "You see my problem, Mr Holmes?"
"Quite an interesting dilemma you have there, Inspector," Holmes mused. "May I?" Upon Gregson's nod, he took the tin, turning it around in his hands, examining it from all angles. "It is a former sweet tin, that seems to have been repurposed for this new cause. Much loved by its owner, and several years old. Beyond that, I cannot tell."
"You can't know if the owner loved it!" Gregson protested, snatching the tin back and examining it for himself. "What, did they scratch a message on it with little hearts?"
Holmes drew himself up to his full height, scowling at Gregson. "If you will not accept the truth of even the simplest deductions, you are perfectly able to leave, and solve this mystery for yourself."
Gregson looked alarmed. "Of course not, Mr Holmes. Just curious, that's all. Seems like more than you can tell from a glance."
Holmes relaxed his posture again. "The scuff marks around the edges of the tin show that it has been much used, and opened time and time again. Why would someone not simply buy a new tin, rather than carry around the old one, unless it was much loved? As for it being a sweet tin, and several years old, I have penned a short monologue on the different types of sweet tins throughout the last decade. This one was last produced four years ago."
"And what of it being repurposed?" Watson asked.
"If there were sweets inside, the weight would be spread more evenly, and it would rattle as it was moved. Instead, the weight is concentrated in one space. Whatever is in that tin, it is no longer sweets."
"So, what, exactly," Gregson asked, "is in that tin?"