Finally, with a mocking casual air, Hekki spoke: "There is a very ancient saying on your planet," he said, "to the effect that bad pennies always return." The corners of his mouth twitched with sardonic amusement. His manner grew more serious, yet still there was an undercurrent of sarcasm: "Miss Darell and Mr. Shelby, I want to compliment you on your remarkable cleverness and daring. Words cannot express my admiration for you. You have every right to be proud of yourselves." Shelby nodded. "We are," he told him drily. "Is there anything more on your mind?" He turned away with an expression of bored contemptuous indifference. "I have little to say except that we are about to continue our recently interrupted journey tonight, Mr. Shelby," said the Martian. He saw the Earthman and the girl casting interested glances at the disc vehicles that surrounded them everywhere. "You like my people?" Hekki inquired. "You find them entertaining? Perhaps you have discovered things in their habits which you cannot understand. Shall I give you explanations?" For the moment at least there was a serious earnest ring in Hekalu's voice. "Flag of truce, Jan. This should be interesting," Shelby said. His eyes were full of eagerness as he turned back toward the Martian. "How do they live out there?" he cried. "There isn't any air or water, and it's almost as cold as it can get anywhere. Why, the thing is utterly impossible according to the laws of common sense!" Immediately all of Hekalu's lazy air of careless mockery was gone, and the dynamic aura of the tireless experimenter and inventor that had hidden beneath it showed out clear. His voice was husky with suppressed excitement when he spoke: "I too was dumbfounded when, some five Earth years ago, I first ran across the Space Men out there. (He waved his hand toward the west away from the sun.) But after I had studied them for a time, I knew that there was really nothing very remarkable or impossible about the nature of their living. It is actually quite similar to our own. "Why do we need air? Simply because by the chemical combination of oxygen with food we obtain the energy necessary to make our brains to think, our limbs to move, and our hearts to beat. Energy is life. But doesn't it occur to you that this vital thing might be obtained in some other manner? The Space Men do. Their principal food is the radio-active element, atomic number 109, as yet undiscovered on the planets. It is a purplish liquid that is fairly abundant on a number of the planetoids. Daily, like radium, it gives off vast quantities of energy; and when in the systems of the Space Men it supplies them with power more efficiently than food and oxygen ever could do for us.