Hurts like a fucking son of a bitch, thanks for asking

This requires a chalkboard. [She sets up her communicator across the room and puts a big piece of butcher paper up on her wall, then grabs a marker.] This'll do.

[She starts drawing a line.] This is one world's timeline. Now imagine a big event, like...the American Revolution. Suddenly there are multiple what-ifs. What if America lost the war. What if they were so weakened by the war that another power, say the French, came in and took over. [She splits off the line into three different branches.] Now we have three worlds. [She puts an x over one of the branches.] Here's the timeline as you know it--America won. Now, imagine that a whole bunch of other what-ifs happen...[more branches] creating more parallel worlds.

Because reality is infinite, every possibility that ever came up is being played out somewhere. There are worlds where we've been conquered by aliens. Worlds where we practice magic instead of relying on science. Worlds where neither of us ever died.

And yes, if I know where I'm going it only takes me long enough to draw up a beer down the pub.