Were long notes on Prof, and I was embarrassed as they added up to "harmless old fool." He was tagged as a subversive--that was why he had been sent to The Rock--as a member of underground group in Luna City. But was described as a "troublemaker" in organization, one who rarely agreed with others.

Prof dimpled and looked pleased. "I must consider trying to sell out and get myself placed on the Warden's payroll." Wyoh did not think this funny, especially when he made clear was not joke, merely unsure tactic was practical. "Revolutions must be financed, dear lady, and one way is for a revolutionary to become a police spy. It is probable that some of those prima-facie traitors are actually on our side."

"I wouldn't trust them!"

"Ah, yes, that is the rub with double agents, to be certain where their loyalties--if any--lie. Do you wish your own dossier? Or would you rather hear it in private?"

Wyoh's record showed no surprises. Warden's finks had tabbed her years back. But I was surprised that I had a record, too--routine check made when I was cleared to work in Authority Complex. Was classed as "non-political" and someone had added "not too bright" which was both unkind and true or why would I get mixed up in Revolution?

Prof had Mike stop read-out (hours more), leaned back and looked thoughtful. "One thing is clear," he said. "The Warden knew plenty about Wyoming and myself long ago. But you, Manuel, are not on his black list."

"After last night?"

"Ah, so. Mike, do you have anything In that file entered in the last twenty-four hours?"

Nothing. Prof said, "Wyoming is right that we cannot stay here forever. Manuel, how many names did you recognize? Six, was it? Did you see any of them last night?"

"No. But might have seen me."

"More likely they missed you in the crowd. I did not spot you until I came down front and I've known you since you were a boy. But it is most unlikely that Wyoming traveled from Hong Kong and spoke at the meeting without her activity being known to the Warden." He looked at Wyoh. "Dear lady, could you bring yourself to play the nominal role of an old man's folly?"

"I suppose so. How, Professor?"

"Manuel is probably in the clear. I am not but from my dossier it seems unlikely that the Authority's finks will bother to pick me up. You they may wish to question or even to hold; you are rated as dangerous. It would be wise for you to stay out of sight. This room-- I'm thinking of renting it for a period--weeks or even years. You could hide in it--if you do not mind the obvious construction that would be placed on your staying here."

Wyoh chuckled. "Why, you darling! Do you think I care what anyone thinks? I'd be delighted to play the role of your bundle baby--and don't be too sure I'd be just playing."

"Never tease an old dog," he said mildly. "He might still have one bite. I may occupy that couch most nights. Manuel, I intend to resume my usual ways--and so should you. While I feel that it will take a busy cossack to arrest me, I will sleep sounder in this hideaway. But in addition to being a hideout this room is good for cell meetings; it has a phone."

Mike said, "Professor, may I offer a suggestion?"

"Certainly, amigo, we want your thoughts."

"I conclude that the hazards increase with each meeting of our executive cell. But meetings need not be corporal; you can meet--and I can join you if I am welcome--by phone."

"You are always welcome, Comrade Mike; we need you. However--" Prof looked worried.

I said, "Prof, don't worry about anybody listening in." I explained how to place a "Sherlock" call. "Phones are safe if Mike supervises call. Reminds me-- You haven't been told how to reach Mike. How, Mike? Prof use my number?"

Between them, they settled on MYSTERIOUS. Prof and Mike shared childlike joy in intrigue for own sake. I suspect Prof enjoyed being rebel long before he worked out his political philosophy, while Mike--how could human freedom matter to him? Revolution was a game--a game that gave him companionship and chance to show off talents. Mike was as conceited a machine as you are ever likely to meet.

"But we still need this room," Prof said, reached into pouch, hauled out thick wad of bills.

I blinked. "Prof, robbed a bank?"

"Not recently. Perhaps again in the future of the Cause requires it. A rental period of one lunar should do as a starter. Will you arrange it, Manuel? The management might be surprised to hear my voice; I came in through a delivery door."

I called manager, bargained for dated key, four weeks. He asked nine hundred Hong Kong. I offered nine hundred Authority. He wanted to know how many would use room? I asked if was policy of Raffles to snoop affairs of guests?

We settled at HK$475; I sent up bills, he sent down two dated keys. I gave one to Wyoh, one to Prof, kept one-day key, knowing they would not reset lock unless we failed to pay at end of lunar.

(Earthside I ran into insolent practice of requiring hotel guest to sign chop--even show identification!)

I asked, "What next? Food?"

"I'm not hungry, Mannie."

"Manuel, you asked us to wait while Mike settled your questions. Let's get back to the basic problem: how we are to cope when we find ourselves facing Terra, David facing Goliath."

"Oh. Been hoping that would go away. Mike? You really have ideas?"

"I said I did, Man," he answered plaintively. "We can throw rocks."

"Bog's sake! No time for jokes."

"But, Man," he protested, "we can throw rocks at Terra. We will."

Took time to get through my skull that Mike was serious, and scheme might work. Then took longer to show Wyoh and Prof how second part was true. Yet both parts should have been obvious.

Mike reasoned so: What is "war"? One book defined war as use of force to achieve political result. And "force" is action of one body on another applied by means of energy.

In war this is done by "weapons"--Luna had none. But weapons, when Mike examined them as class, turned out to be engines for manipulating energy--and energy Luna has plenty. Solar flux alone is good for around one kilowatt per square meter of surface at Lunar noon; sunpower, though cyclic, is effectively unlimited. Hydrogen fusion power is almost as unlimited and cheaper, once ice is mined, magnetic pinchbottle set up. Luna has energy--how to use?

But Luna also has energy of position; she sits at top of gravity well eleven kilometers per second deep and kept from falling in by curb only two and a half km/s high. Mike knew that curb; daily he tossed grain freighters over it, let them slide downhill to Terra.

Mike had computed what would happen if a freighter grossing 100 tonnes (or same mass of rock) falls to Terra, unbraked.

Kinetic energy as it hits is 6.25 x 10^12 joules--over six trillion joules.

This converts in split second to heat. Explosion, big one!

Should have been obvious. Look at Luna: What you see? Thousands on thousands of craters--places where Somebody got playful throwing rocks.

Wyoh said, "Joules don't mean much to me. How does that compare with H-bombs?"

"Uh--" I started to round off in head. Mike's "head" works faster; he answered, "The concussion of a hundred-tonne mass on Terra approaches the yield of a two-kilotonne atomic bomb."

"'Kilo' is a thousand," Wyoh murmured, "and 'mega' is a million-- Why, that's only one fifty-thousandth as much as a hundred-megatonne bomb. Wasn't that the size Sovunion used?"

"Wyoh, honey," I said gently, "that's not how it works. Turn it around. A two-kilotonne yield is equivalent to exploding two million kilograms of trinitrotoluol... and a kilo of TNT is quite an explosion-- Ask any drillman. Two million kilos will wipe out good-sized town. Check, Mike?"