Angels with Dirty Faces

Why is it that the more carbon dust or black grease you have on your hands the more likely it is that you need to scratch your nose?

Got a big job on today doing photographs for a major insurance company. The pics will go in the national press and they are going to pay me shed loads of money. Actually I've got to earn the shed loads of money because not only do they want the pictures taken within twenty four hours but they also gave me a deadline of Thursday lunchtime for the pics to be delivered.
So I shall be working through the night to get the job done.
Not too much trouble for £750.