I adore this series so much! Even as questions are answered, new ones appear! Note: I'm not asking for spoilers, just flailing at the lovely mystery. I love investigating!John.

What is that thing on the roof and how does it relate to Moriarty? Could it hurt John? John's behavior on the roof seems partly instinctual, as well as intelligent (how deep is that "tar"? is its deepness uniform, etc.). So M was contaminating people close to Sherlock (deliberately? and would contamination by Molly or John hurt him?)