What were Sherlock's best deductions in the books?

I know this soalan is very hard but I'm wanting to do an oral at school on Sherlock and I want some of his deductions that are short and easy to understand without having read the story.

I have one which goes something like "Human behaviour leads us to write above eye level" when Sherlock was able to deduce a height just sejak having a person write on the dinding in The Study of Scarlet (He was also able to tell it from how the person walked but I liked the Penulisan on the dinding one better).

If anda guys can help that would be great. Also if anda have any ideas of what would be good to have in an oral on Sherlock, then please say, I'm open to ideas here :).