An ex-police friend of mine, upon enquiry, agreed that a Police Officer only exhibits a form of "Black Humour", generally resulting from some individual's misfortune, either one of their own, or a member of the public.
http://dictionary.reference.com/browse/black+humorThey are trained not to be distracted by a member of the general public attempting to share humour with them, in case a criminal event is taking place out of their sight.
So don't attempt to make a Police Officer laugh with general humour....it rarely works!
After much examination of the species, I mostly keep well clear of them!...

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Paul.....

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Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.