OK Damo, you deserved this. Now were just going to do Engineer jokes.
During WWII a biologist, a chemist and an engineer are captured by the Nazi's for plotting the over throw of the third reich. The Nazi judge orders all three to be shot. The guard takes the biologist out first, lifts his rifle to his shoulder, aims, pulls the trigger ... and nothing happens. The biologist takes off running and escapes.
The Guard goes back and gets the chemist but the same thing happens. He aims the rifle, pulls the trigger and nothing happens, the chemist runs and escapes.
The Guard goes back and gets the engineer and again the same thing happens. He aims the rifle pulls the trigger and nothing happens. The engineer yells, "You idiot, the bolt is jammed, put some grease on it!"