I have a doubt regarding the first season's first episode of Sherlock.
Towards the end, Sherlock says to the serial killer "I know a real gun when I see one".
The serial killer (i.e, the cab driver), actually shows Sherlock the gun, when they arrive at the destination (where the serial killer intends to kill Sherlock). At this moment,Sherlock asks how the cabbie makes the victims follow him and for that the cabbie shows the gun.
At this point, Sherlock tells the cabbie "you don't take people's lives at gun point"
So, my question is, at this point, doesn't Sherlock know it's not a real gun?
Or, is it just intended for the serial killer to think Sherlock is not as intelligent as himself?