One hot summer's day a policeman was standing near a street corner in a town. A tram-line ran along the main road in front of him, but the trams did not pass very often.
While he was standing there, the policeman suddenly noticed a small old lady who was behaving very strangely. Although there was no traffic in the road, because it was about two o'clock in the afternoon, and everybody was at home resting, she was suddenly running into the middle of the street, then stopping, jumping up in the air, and then running backwards to the side of the street again.
The policeman watched her for a minute, and then walked towards her. 'What's the matter, madam?' he asked. 'There's no traffic on the street, so you may cross now without any trouble.'
'Oh, I'm glad that you've come,' said the old lady in a trembling voice, 'I'm a stranger here. I live in a small town, where we have no trams, and I don't know how to cross the lines without danger.'
'Madam,' said the policeman, 'you needn't worry. When no trams are coming, you can cross quite safely'
'But the trams run by electricity, and if I put my foot on one of those steel tram-lines, the electricity will kill me! I'm old, and my legs are stiff. I'm afraid that I may fall on one of the lines when I try to jump over it.'
'Madam,' answered the policeman, 'do you see that steel wire about twenty feet above the tram-lines?'
'Yes,' answered the little old lady.
'Well,' said the policeman,' unless you touch that wire with one of your feet, and at the same time touch one of the rails with your other foot, the electricity can't hurt you.'