The easiest way to do this is by telling them a story, and one story that has never failed to please is how I once caught a London bus.

   I had been staying with friends who live in the centre of London, and one morning I had made up my mind to visit Harrods, which is in Kensington.
   My friends urged me take an Oxford Street bus, telling me that this is - and it still is - the most convenient way to travel around central London. I was told the number of the bus; the bus stop at which to stand in order to catch it - apart from tying a label around my neck with my name and address, and giving me a package of sandwiches, there was nothing more they could do to speed me on my way.

   I walked across to Oxford Street - this early in the morning it was surprisingly quiet - and I stood patiently at the bus-stop for fully ten minutes as big red double-decker bus after bus rumbled past. All were the wrong ones. Finally I decided to walk.
   I had walked only fifty yards when behind me I heard the unmistakable grumble of yet another bus.
   Turning to look, I realised that this must be my bus! It was now rather less than fifty yards away, and it was accelerating hard. In the clear morning air with sunlight flooding across Oxford Street from the south I could see that it was empty.
I could also see the driver, and by the way that he was holding the wheel it was clear that he would be happy to keep it empty. I might wave at him in vain. The daily life of a London bus-driver, endlessly stopping and starting, is not easy. With an empty bus and an empty road in front of him, he had obviously no intention to stop for one stupid passenger who was fifty yards past an official stop.
   There then flashed into my mind in this moment - in, I suppose, about two seconds - the complete and exact description that I had read just a few months before of the method taught to members of the French Resistance sixty years ago by the Special Operations Executive, the SOE, for catching trains between railway stations.
    In those days, especially if you were an agent, this was an extremely useful precaution, because all the stations were controlled by the French police and the Gestapo, the German secret police. Either one - or both - would carefully check any traveller's identification papers.
   To avoid any inconvenience that might arise from this - like being shot - the agents were taught to wait beside the railway line at night until they heard a train approaching. The method they then used was possible - is possible - with any kind of train, either passenger or goods, but a goods train was best because it would be less likely to have guards aboard. Once one got into a goods wagon, one could expect to be relatively safe for many miles. It would almost always also stop in a goods yard: there might be less security there as well.
   The agents had stand with their toes on the edge of the railway sleepers - ties, Americans call them - so that as the train passed them it would be only inches away from their body and their face. Then they were to bend sideways, in the direction the train was travelling, and hold out their arms above their head - rather like a ballerina in Swan Lake, but with their palms flat as if they were about to pat the side of the train - and then they were to move their hands forward until the projections of the moving train were just brushing against their fingers.
   The main idea was this. At the front and rear of every carriage, also on almost every wagon, was a vertical metal handle. Below both was a step. To get up into the carriage, or to climb onto the wagon, people would take hold of this handle with one hand, and pull themselves up onto the step. That was what they did when the train was not moving.
   Standing there in the night, on their toes - with the side of the train roaring and crashing past their faces just inches away, and those great steel wheels hissing and banging below - the agents would incline further and further forward until he, or sometimes she, could feel these vertical handles flicking past under their outstretched fingers; and then, at the very last moment, they would hook! their fingers, catch the last of the handles, and the huge momentum of the train would simply pluck them off the sleepers, and they would come down safely - hoopla - onto the step.
   Nothing, in principle, could be simpler: un morceau de gateau!
   All of this travelled through my mind in a couple of heart-beats. By now I could see the driver's face clearly through his almost vertical window. I could see his arm move. I heard the engine note drop as he moved determinedly up another gear. He was deliberately looking far beyond me. I did not exist for him.
   Of course there were no sleepers to stand on here, but I could see that the big tyres of his bus would pass within two feet from the kerb. The bus might have been on rails itself. Stepping down from the kerb, I stood now so that the tyres would just miss my toes, and assumed the precise position required by the Special Operations Executive. (At this point, incidentally, before a fascinated class, I would assume this position. Demonstration is a lesson in itself.)
   The front of the bus roared past. The driver's face, flashing by, looked sideways through his little window. He looked amazed. The great smooth flat red side of his bus is now sliding past my fingers. How frightening to do have to do this at night, in the dark, when any unexpected unseen projection might kill you at once: throwing you aside like so much rubbish. That took courage. The air dragged by the bus is lifting my hair. I can feel the lines of rivets in the red metal panels flicking my fingers.
   Now! Hook on!
   In those days there was a platform at the rear of all London buses for passengers to get on and off. The conductor of bus - he took the fares and gave out tickets from a little winding machine - would often stand there staring at the traffic. In the middle of this platform's outer edge was a long chrome bar reaching from the floor to the ceiling. (With a young class I would sometimes stop at this point and draw a bus quickly to show them.) This bar was more or less where the handle would have been on a French train, whether passenger or goods. This is what I hooked on to.
   An empty London bus has a weight, I should say, of about ten tons. It cannot accelerate all that quickly, and even now it was probably only travelling at a bit more than twenty miles an hour. There was a tremendous jerk on my arms, and a very loud twong that echoed through the empty bus like a plucked string on a big bass guitar, and I was suddenly travelling down Oxford Street, horizontally, pulled behind the bus rather like a solid flag. With legs.
   The driver was still accelerating hard. He had apparently decided to leave behind the idiot whose toes his tyres had just run over. What he had not yet realised was I am now his only passenger. He cannot see me. I am still airborne, for I have not yet got my feet down on the platform, and I am still hidden from his view by the side of his bus.
   I am surprised to find that I am still horizontal, although the reason that I am still horizontal - and if I had only had another second to think about it, I might have expected this before I executed my plan - is simply physics.
   I weighed about twelve and a half stone, around one hundred and eighty pounds, or 70 kilos. This mass - that is, me - cannot be accelerated instantaneously to the same speed of the bus. Acceleration takes time, however short, in which a force, however large, has to act to give a mass the kinetic energy it requires to move..
   That first deep twong - now left behind in our wake - was the sound of this force beginning to act on the mass - that is, me. I am horizontal because my arms were parallel with the ground and that is the direction the force is acting, and I am alarmed to see that the bar - it is now, of course, above my head - is bending like a bow with the strain. And now, just as if am on a trapeze, I begin to swing, very rapidly indeed, in behind the bus, and this is how I discover - only by doing it myself, you see - why the SOE advised their agents to catch hold only of the very last handle on the train. The reason is that then there is nothing more behind the train for them to hit.
   The force that has just lifted me off the road is now also causing me to rotate. I am now not only horizontal. I am also now rotating towards the back of the bus because my centre of mass (pointing) was not directly behind the bar when the force began to act. Now any body, as you know, that is once made to move by some external force in any particular direction, or that is made to turn about its axis, will continue to move or turn in that direction until it is acted upon by some other external force. This is Newton's First Law.
   On this older type of London buses there was a second bar for the convenience of the passengers. This is shorter, also chromed; but is much sturdier than the one that I am still holding. It is just to the right of the platform, at the rear, and I now collide with this bar with a frightful thump. That is the new external force. It stops me rotating.
    My horizontal speed is now the same as the horizontal speed of the bus, so that I am also now dropping down onto it. I still have the presence of mind to bring my feet down under me, to land on the platform - just as the conductor, he has a black moustache bristling in his white face, comes clattering down the stairs like a sack of coals in a dark blue London Transport uniform.
   He must have seen everything from the upper deck. Even from that perspective it must have been impressive.. First this solitary pedestrian is just ambling along the pavement. Suddenly he turns, looks up, making a sudden, terrible decision and throws himself towards the bus, steps into the road, disappears under the wheels. A suicide!
   Now, discovering the suicide is standing on his platform, alive and well, not left behind in Oxford Street like a squashed tomato - and he had looked swiftly behind to make sure there are not two maniacs about this morning - he is remarkably unrelieved. He is instead extremely angry.
   "What the fuckinell do you think you're doin'!" he yelped, making no attempt to praise my intrepid spirit, still less to give thanks to God. He was holding his ticket machine with one hand, the other was over his leather money bag - as if I might now make a spring for a ticket or his cash. These days, his expression revealed his thoughts: you just never bloody know! *
   Meanwhile I am acutely embarrassed. Temporarily, I discover, I cannot speak. There is a really terrible pain in my side. Still holding onto my bar for support, I am bent forward from the waist, whooping for breath.
"   Unnghh," I manage finally, and then, and always think that this is so typically English: "I'm - umm - sorry. I just want a ticket - unghh - to - aaaarrods. Aarunngh. Please."
   "Well," said the conductor with great satisfaction, "then you can just gerrorf this bus, mate. We're goin' to Swiss Cottage."
   And he binged his bell; the bus ground to a halt; I gorroff - just as he advised; he binged his bell again, and as his bus roared away he stood on the platform staring back at me; he was shaking his head unsympathetically, and making rude gestures with his free hand.
   Of course, the SOE also taught its agents how to get off a moving train: so that when they arrived, at wherever they happened to be going, they would not have to explain to anyone where they had come from.
   This is another relatively simple technique. It involves throwing yourself sideways from the train, if possible down an embankment - best of all if it has slowed down on an incline. I didn't think of this at the time - and, anyway, the bus had already stopped to let me off.
   Now, if you'll all stop laughing - when you have all stopped laughing - we'll get on with some work. One day I must tell you about the Hotchkiss Ambulance and the Terror Ride of Grafenwöhr. That might be next week.
Now, all open your books."

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