One of my favorite deductions by Sherlock Holmes is the one where Sherlock tells the life of Watsons brother by examining Watsons pocket watch In the Novel The Sign Of Four. This deduction was cleverly modernized in the BBC television series "Sherlock", where Sherlock did the same thing only with Watsons cell phone.
POCKET WATCH SCENE FROM THE SIGN OF FOUR -----------
Sherlock- "I began by stating that your brother was careless.When you observe the lower part of that watch-case you notice that it is not only dinted in two places but it is cut and marked all over from keeping other hard objects, such as coins or keys, in the same pocket. Surely it is no great feat to assume that a man who treats a fifty-guinea watch so cavalierly must be a careless man. Neither is it a very far-fetched inference that a man who inherits one article of such value is pretty well provided for in other respects."
Watson- "I nodded to show that i followed his reasoning."
Sherlock- "It is very customary for pawnbrokers in England, when they take a watch, to scratch the numbers of the ticket with a pinpoint upon the inside of the case. It is more handy than a label as there is no risk of the number being lost or transposed. There are no less than four such numbers visible to my lens on the inside of this case. Inference- that your brother was often at low water. Secondary inference- that he had occasional bursts of prosperity, or he could not have redeemed the pledge. Finally, I ask you to look at the inner plate, which contains the keyhole. Look at the thousands of scratches all around the hole-marks where the key has slipped. What sober man's key could have scored those grooves? But you will never see a drunkard's watch without them. He winds it at night, and he leaves these traces of his unsteady hand. Where is the mystery in all this?"