A police sergeant was training three blonde recruits in observation and detection. He called the first blonde in and said, "I'm going to show you a photograph of a suspect for a few seconds then, when I remove it, I want you to tell me how you would recognise the suspect in future". After he had removed the photo the blonde said, "We'd soon catch him, he's only got one eye". "Look", said the sergeant patiently, "the photo of him is taken from the side, in profile, of course he's got two eyes".

He called in the second blonde, repeated the procedure with her and, when he'd removed the photo, she said, "That's easy, we'd trace him in no time because he's only got one ear". The sergeant said, somewhat testily, "Don't be silly, the photo is of him in profile, from the side, of course he's got two ears!"

He summoned the third blonde, repeated the procedure, then said, "I'm now going to remove the photo, I want you to think carefully and don't give me any stupid answers". The girl pondered for some time, then said, "We would be able to identify him because he's wearing contact lenses". "That's amazing", said the sergeant, "the suspect does indeed wear contact lenses but how could you deduce that when it's not obvious from the photo?" "Well," replied the blonde, "I knew he couldn't wear ordinary glasses because he's only got one eye and one ear!"