In fact, if you're not watching it, you should probably miss out on this blog post too. Seriously, stop reading.
What follows is an in-depth, full-of-spoilers discussion of what happens in the final episode of the show thus far, The Reichenbach Fall.
The final question
I'm sure you know exactly what I'm going to discuss: how did Sherlock fake his own death?
Using my amateur deductive reasoning and a healthy amount of rewind & pause, I have a pretty good guess at what happened. Let me walk you through the reasoning.
To start with, let's state the very obvious: either the body that fell off the roof was (a) Sherlock, or (b) it wasn't. I think we can fairly confidently eliminate option (b).
Could Sherlock have thrown Moriarty's body or the test dummy from earlier in the episode in his place? Not likely.
First, there was no way Sherlock could have bent over, hauled up a body or a dummy, brought it to the edge, and shoved it over, all without Watson noticing. There were some quick cuts and edits during the scenen, but we have no reason to believe that Watson looked away from Sherlock at any point during their conversation.
Other practicalities make this even more difficult: Moriarty was dressed noticeably differently than Sherlock (different coat, white shirt instead of dark, a tie instead of a scarf) and had shorter hair; seconds before Sherlock jumps, he looks back and Moriarty's body is lying there, still in the original outfit; the scene where Sherlock jumps is shot from behind and it's clear no one is just shoving a body/dummy off the roof.
Verdict: it must have been Sherlock himself who jumped off the building.
How did he survive?
One key piece of evidence is that Sherlock is extremely specific in where he wants Watson to stand during their conversation. Here is the layout of the scene:
When Watson arrives, Sherlock is on the hospital roof, some 6-8 stories up, with a shorter, 2-3 story brick building between him and Watson. Watson tries to run around to the side of the brick building, but Sherlock yells at him to return to his previous spot. Sherlock is very vehement about this.
It's likely that Watson would be able to see Sherlock from either vantage point, which leaves only one other possibility for why Sherlock would care about where Watson stands:
(1) There was something between the brick building and the hospital that Sherlock didn't want Watson to see.After Sherlock jumps, Watson again tries to run around the brick building and this time, is knocked down by someone on a bike. This is unlikely to be an accident and gives us our second hint:
(2) Sherlock needed to delay Watson until the something was no longer visible.If you watch the scene closely, there is one item that fits both of these criteria: a truck filled with bags (garbage? recycling? laundry?) parked right next to the spot where Sherlock's body ends up. You get your first glimpse of this truck just as Watson is coming around the corner, just before he is knocked down:
You see the same truck drive away, out of the scene, a few seconds later as Watson finally gets to Holmes' body:
Think on that for a second: if a body comes crashing down a few feet from your truck, do you just casually drive away or jump out and see what the hell just happened? The fact that the truck drove away increases our confidence that it was part of the plot.
Verdict: the bags in the back of the truck served as padding to break Sherlocks' fall.
How did it go down?
Before meeting with Moriarty, Holmes seeks out Molly and tells her that he thinks he will die and that he needs her help. Holmes must've already realized that Moriarty's goal was to get him to commit suicide, so he enlisted Molly - who works at a morgue and could certainly fake autopsy reports and death certificates - to help him fake it. It's also worth remembering that it was Sherlock, not Moriarty, who arranged the meeting on, of all places, the rooftop of a hospital.
When Moriarty blew his brains out, Sherlock had no choice, and jumped. He landed in the truck, covered himself with some sort of blood (possibly provided by Molly), and dropped down onto the pavement to play dead. In fact, he did better than that. We saw Holmes with a bouncy ball much of the episode; it turns out there is a classic magic trick that involves squeezing a ball under your armpit to cut off circulation to your arm and make it seem like you have no heartbeat.
What about the bystanders?
Since all the bystanders could see the truck and Holmes fall into it, they must have been in on it. The crowd that gathers around Holmes' body and the biker that knocks over Watson were either part of Holmes' homeless network or government folks brought in by Mycroft.
Mycroft is an interesting possibility because his reaction to reading about Holmes' death is ambiguous: was he sad or relieved? Even more telling is the fact that Mycroft isn't with Watson and Mrs. Hudson at the cemetery to pay his respects to Holmes. My guess is that Mycroft knows Sherlock is alive, though it's possible that he merely deduced it after the fact.
No matter how improbable..
It'll be some time before the third season comes out and reveals the truth. In the meantime, feel free to join me in speculating by leaving your best theory in the comments.