He lunged at Sean, succeeded only in tearing that metal contrivance from his back. A great weight suddenly pulled Sean to the pavement, seemed to triple the weight of his own body. It was pain to move his head, but Sean's red-thatch twisted so his green eyes could see. The pursuing Krak toppled against the black bricks beside Sean, his bald head making a dull sound. The usually impassive eyes were staring at Sean's green orbs. There was pain and--was it defeat?--in them. Every sinewy muscle in Sean's body strained as he tried to get to his feet. So that was what the metal pack was for, he decided irrelevantly, an anti-gravity device. He threw his body toward it. Before he reached it, however, Mike had picked it up, was strapping it haphazardly on his back. The tremendous weight lifted and he crawled to his feet. "You were right after all," Mike said, and there was a caress in it. "Laughter." Sean stood a long moment, looking at the fallen Kraks. Sean began to chuckle, the chuckle drifted into laughter. It was true! Humanity had forgotten its greatest weapon. "God," said Mike softly. "Laughter did it. Laughter." His dark eyes were staring at Sean. Then he, too, was laughing, joining his bass with Sean's baritone. * * * * * Earthlings moved out of the laundry, their eyes wide. They, too, fired by the infectious roarings began to laugh. On the wings of the wind, the laughter spread, working its way building by building, street by street, block by block through the city, as other humans picked it up, flung it on joyfully. And as the Earthly laughter bubbled and rolled through the sullen city of Karrar, Kraks died--only a few at first, but more and more as the bursts of laughter swelled and swelled until even the black and red stone echoed with it. Mike O'Hara placed his big hand on Sean McKenna's arm. "You found the chink, Sean," he said. "Was it the sound of the laughter? That doesn't sound right." He chuckled a little at the unexpected pun. Sean grinned. "I know what you're driving at, Mike. Laughter is scaled so low on the vibration scale that the Kraks must have encountered other vibrations of the same intensity at many times in the past. That it?" Mike nodded. Sean grinned impishly. "Laugh once, Mike, and listen to your laughter." Mike laughed, his brow furrowed. "No idea bloomed," he said when he stopped laughing. "Burlesque it," Sean said. "Do it in slow motion." He demonstrated. "Like this. Ha--ha--ha--ha." "Got it!" Mike exclaimed. "It's not a single sound. It's a series of them. It's the old story of the soldiers crossing the bridge. It's not each individual soldier; it's the cadence. Not ha, but ha--ha--ha." "Like kicking at the lock of a door instead of pushing on it steadily to get it open; like chipping at a rock instead of trying to smash it with one blow--there's a slough of analogies if we wanted to go on with it." "That one Krak muttered something about his ears," Mike put in.