One sure way to live dangerously is to become a practical joker.Should you have any doubts about it you might ask Professor Dane.

You didn’t have to be a potential Einstein to take Professor Dane’s course. For one thing you got a few easy credits and for another you were entertained-without letup-by Professor Lyman Dane’s celebrated wit.

Take the time he was illustrating terminal velocity. He jumped out of the open third story window, horrifying the class, until they learned he’d rigged a canvas life net on the floor below. Or the time he let a mouse loose among the female students to illustrate chain reaction. Or the afternoon he played boogie-woogie on the Huyler Memorial Carillon.

“The absorption of knowledge,” he used to say, “increases in direct proportion to the sense of humor-the belly laugh, measured in decibels, being constant.”

He could say a thing like that and make it sound funnier than anybody else could. It was partly the way he looked-tall and mournful and sly, with wispy hair that had once been blond, drooping like a tired willow over his forehead.

But for all his vaudeville tactics he was by no means a second-rate scientist. Which was why he had gained his position at Southwestern Tech in the first place. He refused to work directly for the government (no sense of humor, just initials, he said) but this way he could at least be called upon for consultation at the nearby Air Force Development Center, just at the foot of the mountains to the west.

Now the AFDC, as it was called, didn’t advertise what sort of thing it was developing-but everybody knew that Lyman Dane was an expert on reactive propulsion of rocket motors. He could tell you-and frequently would without being asked-exactly what mass ratio, nozzle diameter and propulsive velocity would be needed for the first trip to the Moon. He knew how many hours a round trip would take, both for landing there or merely circling the body of the satellite.

He had the courses to Mars and Venus thoroughly charted-but considered a trip to Jupiter somewhat impractical. So, what with Dane’s presence and the mysterious white streaks that so often shot up into the sky like fuzzy yarn from the AFDC base, it wasn’t hard to guess what was going on.

Nevertheless Professor Dane was surprised and somewhat offended when the young man from the Federal Bureau of Investigation came to call on him one afternoon. And the worst part of it was that the young man didn’t have much sense of humor.

“As you know, sir,” the young man said, “we’ve been sighting and tracking these unidentified objects in the sky. You must have read about those they chased near Atlanta yesterday.”

“Ah,” said Professor Dane. “Martian through Georgia, no doubt.”

The young man stared at him blankly. He seemed to Professor Dane one of the most nondescript young men his eyes had ever beheld. He had a clean-shaven, pleasant face without exactly being handsome and his eyes were sincere and mild. He wore a neat gray tropical worsted suit and an unobtrusive tie. He was about thirty. Professor Dane supposed that all this was an advantage in his profession.

The young man went on-earnestly. “Without forming any theories about these things we’ve been asked to take certain precautions. I don’t know whether they suspect a hostile power, or what. That’s not my job. At any rate I’ve been given the responsibility of instituting certain security techniques. You do after all, sir, have access to and knowledge of considerable classified information.”

This lad reminded him somewhat of his old friend and colleague, Dr. Fincher, out in California. Wally Fincher was a well-known physicist now, though how anyone ever managed to struggle through his dry ponderous books Dane didn’t know. Probably he had gained most of his fame through his part in those experiments where they bounced radar blips off the moon, Dane thought.

Wally always talked in long unnecessary words. He never merely “went” when he could “proceed,” he never simply “used” when it was possible to “utilize,” he didn’t “get things done”-he “implemented” them. Professor Dane made a mental note to put in a long distance call to Wally that evening and tweak his nose a bit. Maybe Dane could pretend he was the FBI-disguise his voice and interrogate Wally, as though he were investigating him. He chuckled a little at the idea. Then he realized that the young man had been talking and he hadn’t been listening.

“... so among other things, sir, we thought it best to monitor your official mail and hope you won’t mind.”

“What?” said Dane, raising his eyebrows.

“And your phone. You’ll hear a couple of clicks whenever you use it. We’re recording what’s said over it-though I assure you all records obtained will be kept in strictest confidence.”

Dane acquiesced. The young man finally managed to make it clear that all this surveillance would have to be with Dane’s permission and the professor, annoyed though he was, didn’t want to appear uncooperative. He couldn’t resist, however, giving the young man the wrong hat when he went out and being delighted when the young man came back for the right one five minutes later. He was glad to see that something could fluster him.

But that wasn’t really enough. Professor Dane had been annoyed, and he needed to express himself further-by means of the joke, which was his art-in order to regain some measure of his equilibrium and self-respect.

Inspiration visited him as he was climbing the stairs to his bedroom at ten-thirty that evening. He stopped short, thought a minute, then began to chuckle. He turned and went downstairs again, stepped to the phone. Professor Dane lived alone and no one else would be able to share his planned joke-but this didn’t matter.

He had been privately enjoying his pranks ever since, as a frail boy with an unreasonable and dominating male parent, he had discovered that they were one way in which he could compete with hardier souls, at times even surpass them. Never mind the audience, he thought. The jest was the thing!

It was an hour earlier in Los Angeles and Dr. Wallace Fincher was at home. Dane disguised his voice-he did a lot of University Theater work and this kind of thing came to him easily. He listened first to Dr. Fincher’s arid, humorless, “Hello. Dr. Fincher speaking.” Then he heard the preliminary clicking, just as the FBI man had predicted.

“Thandor,” said Professor Dane, “this is Klon calling.”

“I beg your pardon?” said Doctor Fincher.

“The jig’s up,” said Professor Dane. “Captain Ixl in propul-cruiser nine-nine-seven-three will never be able to break through. The Earthlings have set up a close watch-they’re suspicious.”

“Who is this?” Doctor Fincher sounded startled. “Who the devil is this calling?”

Dane could barely keep his laughter from breaking into his voice. “Thandor, we can come to no conclusion but that the Terrestrials are definitely hostile. We should have expected that from their primitive stage of development. They have orders to shoot any of our propul-cruisers they can catch. I suggest that we withdraw all ships of the Franistan class immediately from their free orbits and send them on a standard Keplerian course to the home planet for further consultation.”

“Is this some kind of joke?” Fincher sounded as if he were almost panicky.

“Furthermore,” said Dane, “I recommend that we withdraw all agents from Earth. We can’t conceal our superior mental development and advanced technology much longer.

“Someone’s bound to catch on pretty soon. I was against this plan in the Galactic Council in the first place, you’ll remember. Well, farewell, Thandor! I’ll be seeing you soon in space!”

And Professor Dane hung up before he exploded with laughter.

He laughed until the tears came to his eyes. He held his stomach with both hands. He was weak. He supported himself on the stair railing and for minutes was unable to take the first tread. With his lively scientist’s imagination he could picture the completely bewildered look on the young FBI man’s face when he listened to this conversation on the tape recorder or whatever it was they used.

He was certainly going to have to try to get that recording from them. Play it back for Fincher some time-Lordy, Fincher would have apoplexy every time he heard it!

He finally gained enough strength to climb the stairs. He went into his bedroom, still chuckling weakly, still wiping the tears from his eyes, stomach muscles still aching.

Dr. Wallace Fincher stood there by his bed. It was Fincher-the same stocky round-faced man with the steel-rimmed glasses he had always known. It was either Fincher or the darndest hallucination he had ever ...

“I’m sorry, Lyman,” said Dr. Fincher in a kindly but impersonal voice. “You were getting a trifle too close. I’m afraid you have left me no choice.”

He pointed a little silvery tube at Professor Dane and there was a soft buzzing and the smell of ozone and Professor Dane was no longer in the room-or anywhere else.

Dr. Fincher sighed, adjusted his glasses and faded into the dimension that would take him back to Los Angeles and his interrupted work.