What I always say is, reciprocation is everything. Well, actually, I've never said it before this year, and when I said it to Gaye she just looked at me and asked if it was supposed to be a joke. Which of course it was, but she might at least have played along, and helped me tweak the pun. I'm convinced there's a really neat witticism in there somewhere. You know, reciprocation equals mutuality and Christmas giving and all that, but also motion back and forth in a straight line, like a reciprocating engine. And fusion equals both social cohesion and an exothermic thermonuclear reaction. So reciprocation helps you fuse. Get it?

All right, but this year Uncle George has been working on his new "ping pong fusion" technique, using a fusile plasma compressed and super-heated between inertially driven converging magnetic fields.

You probably remember how Gaye blew up the turkey and got Uncle George interested in micro-fusion. What they're now calling sonofusion, I hear, though Gaye and Uncle George called it bubble fusion, which is catchier. Sonofusion sounds more like some super-tweeting hi-fi loudspeaker or a cinematic special-effects company, which come to think of it aren't so far off. Unfortunately, Uncle George couldn't get it to work. Oh, he could generate any number of fusion neutrons, but nowhere near break even.

"Besides," he said, "bubble fusion is intrinsically small scale. A few kilowatts tops. It's never going to replace a proper power station."

Like Gaye, he hates to admit defeat. Last time we played chess I got him with a fool's mate, and he tried to wriggle out by claiming he wasn't playing chess, he was playing draughts with chess pieces because Gaye was using the draughts pieces as counters in a numerical simulation. When I pointed out to him that his opening moves had been chess moves, not draughts moves, he said he'd got confused and sulked the rest of the day. He even lit his pipe indoors in his study, which he normally never does, and smoked endless pipefuls until Gaye threw open the french windows and pushed him out into the garden. She wasn't really mad at him, but at times like this she feels she has to be mother. I'm glad she does, actually; even a Gaye-mother is better than no mother at all. He'd never admit it, but I'm sure Uncle George feels much the same, which is why he lets her bully him.

Anyway, if bubble fusion wasn't popping, some other sort of fusion would do just as well, or even better.

You're probably wondering how he came to think of ping pong fusion … What is ping pong fusion? … I'll explain later.

One afternoon, when it was too wet to play outside unless you wanted to get really wet, which we didn't, we were playing table tennis in the den and being a bit silly. It was love-all for about the fiftieth time. Neither of us seemed able to clinch the game. First Gaye would sneak one past my guard, and Uncle George would intone "Advantage Gaye", then, expecting to lose, I'd smash one hard back, clipping the table edge in an unreturnable volley. Then my next stroke would just touch the top of the net and flop dead onto Gaye's side. "Advantage the sprout on my left," Uncle George would go, and while I was putting my tongue out at him and formulating a suitably rude reply, Gaye would promptly equalise with a neat shot to the opposite side of the table.

It went on like this for ages, fluke after fluke, volley after volley, until it seemed we couldn't miss and the ball flew back and forth like … like something that bounces very fast from side to side … like a reciprocating engine, in fact. We drew in closer and closer together. The ball ricocheted faster and faster between the bats … ping, pong … ping, pong … ping pong … ping pong … ping pong, ping pong, ping pong, pingpong, pingpong, pingpong.

The bats moved even closer, and suddenly there was no more time for both the tap on the bat (the ping) and the bounce on the table (the pong), only the pang of a direct volley from bat to bat, faster and yet faster, until bats and ball met in the middle in a loud singularity like something from Aesop's fables or Zeno's paradoxes. Pingpong, pingpong, pingpang, pingpang, pang, pang, pang,pangpangpangprrrrap!

The ball burst into pieces and sprayed itself across the room.

Uncle George sneezed a piece out of his nose. "By George!" he said. "That's interesting."

"Not really," said Gaye, "It's just snot."

"Shut up, Gaye," I said. "Whose point was that?"

"Good question," said Uncle George. "Whoever hit it last, I suppose, out of an infinite number of volleys. Consider it as the inverse of the Chinese proverb of the sound of one hand clapping."

"Devilish clever these Chinese!" we chorused.

"But isn't Zen Japanese?" I queried. "And … I say … do you think they're related to Zeno?"

Uncle George looked blank.

"It sounds the same," I explained.

"It couldn't really have bounced an infinite number of times, could it?" asked Gaye.

"Well, nearly infinite," said Uncle George. Gaye snorted. "Lots, anyhow," he amplified. "It must have been going at one heck of a lick near the end."

"Supersonic!" I said, but Uncle George took me literally.

"Probably. If the bats are closing with velocity v then for an elastic collision each impact increases the ball's speed by 2v."

"Whips are supersonic," agreed Gaye. "That's why they go crack."

"You know," mused Uncle George, "I was reading the other day how the same sort of thing happens with the solar wind and extra-galactic radio sources. When a bow shock ploughs into a stationary plasma, or vice versa, it heats it up and accelerates some of it to ridiculously high energies. That's where the radio emissions come from; energetic charged particles spiralling in a magnetic field."

Gaye nodded knowledgeably. "Synchrotron radiation."

"And bremstrahlung," I added. "Well, I read, too!"

"If we had magnetic bats we could play ping pong with plasma," said Gaye.

"Just what I was thinking," said Uncle George. "And what happens when you squeeze very hot plasma very hard?"

"Bang?" suggested Gaye.

"Exactly!" said Uncle George. "It undergoes thermonuclear fusion."

"Like the Sun," I said, to show I was keeping up.

"Exactly," said Uncle George again. "Only more like a supernova, where the fusion is driven by the collapse of the core."

"Neat," I said. "Can you do it in the lab?"

"You know, I think perhaps I can!"

So he did.

Without divulging too many proprietary details — which Uncle George expects to make him ultra-stinking rich, though he's already pretty well rolling — it goes like this. There's a metal and ceramic tube standing on end like a mortar, and a pair of massive tungsten slugs sliding up and down inside like pistons — there's reciprocation for you! Thick coils round the outside create a magnetic field, which gets squeezed alternately between the pistons and the heavy end plates. It's the magnetic field that flings the pistons back and forth. In the gap between the pistons, it's mainly vacuum, with some diborane gas to make the plasma. Uncle George says that's the best fuel because it fuses without producing neutrons or gamma rays, so he can work on it without having to use yards of lead and concrete shielding. It's just like an internal combustion engine, really — a car motor or electric generator — except that the power is tapped directly from the coils as the magnetic field sweeps through them, twice a millisecond cycle.

So what, I hear you ask, does this have to do with Christmas? Not a lot, but it was Christmas when his first experimental set-up successfully met the Lawson Criterion (which is when a plasma stays hot enough and dense enough long enough to go bang). It was a simpler one-shot arrangement in which the tungsten slugs were propelled together by explosives.

Finishing too late on Christmas Eve to test it, Uncle George left the primers in place overnight. Afterwards, he blamed Gaye and me for interrupting him, but it was his own fault really. Even so, nothing much would have happened until the day after Boxing Day, when Uncle George meant to get back to work, if it hadn't been for Gaye.

We'd just finished the Christmas turkey, carefully cooked by me in the oven, not the washing machine, and had made a start on the Christmas pudding, with white sauce carefully prepared by me out of a packet, and brandy carelessly poured on his by Uncle George out of a bottle, when Gaye decided she couldn't wait any longer to open at least one Christmas present. The one she chose was one of those wrapped up by one of those infuriating people who think a present isn't wrapped up without a forest of wrapping paper and a full roll of sellotape. Personally, I always remove sellotape delicately with my finger nail so the paper can be re-used, but Gaye's more of the dig in, heave and tear type.

All at once, the tortured wrapping gave up the ghost, a tin of talcum powder decorated my half-eaten Christmas pud, and a bar of soap hurtled across the room, knocked the Christmas tree off the telly, fused the fairy lights and sent a voltage spike winging through the mains to the lab, triggering the explosives and the ionising arc through the plasma.

Break even was achieved as half the compressed and overheated boron fused in a single instant, blasting the tungsten slugs back in their tracks with considerably more energy than the chemical propellants had imparted on the way in.

One slug dug a deep hole into the ground, the other removed the laboratory roof and departed into the outer stratosphere. Having, in a single sharp detonation, brought good tidings of great joy to Gaye and Uncle George, which they hope shall soon be to all people, except those owning obsolete power stations, and leaving on Earth, pieces and good will toward men, or at any rate a warm glow, it was gone away into heaven like the angels (Luke 2 vv 10-15).

The abrupt and unexplained failure of one of the constellation of GPS satellites whose ephemeris took it overhead at that time on Christmas Day may have been coincidental. It would have made a very small target. Really. Do the sums.

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