"The Nazis lost a lot of planes that way," said Talley. "Those that didn't crash from stuck valves in flight--they had to have their valves reground. Lost flying time. Wonderful! And when the Nazis did uncover the trick, they had to re-refine every drop of aviation gas they had!" Joe said: "That's it!" "That's it? And _it_ is what?" Then Joe said disgustedly: "Surely! It's the trick of loading CO_2 bottles with explosive gas, too! Excuse me!" He got up from the table and hurried out. He found a phone booth and got the Shed, and then the security office, and at long last Major Holt. The Major's tone was curt. "Yes?... Joe?... The three men from the affair of the lake were tracked this morning. When they were cornered they tried to fight. I am afraid we'll get no information from them, if that's what you wanted to know." The Major's manner seemed to disapprove of Joe as expressing curiosity. His words meant, of course, that the three would-be murderers had been fatally shot. Joe said carefully: "That wasn't what I called about, sir. I think I've found out something about the pushpots. How they're made to crash. But my hunch needs to be checked." The Major said briefly: "Tell me." Joe said: "All the tricks but one, that were used on the plane I came on, were the same kind of trick. They were all arrangements for getting regular destructive items--bombs or rockets or whatever--where they could explode and smash things. The saboteurs were adding destructive items to various states of things. But there was one trick that was different." "Yes?" said the Major, on the telephone. "Putting explosive gas in the CO_2 bottles," said Joe painstakingly, "wasn't adding a new gadget to a situation. It was changing something that was already there. The saboteurs took something that belonged in a plane and changed it. They did not put something new into a plane--or a situation--that didn't belong there. It was a special kind of thinking. You see, sir?" The Major, to do him justice, had the gift of listening. He waited. "The pushpots," said Joe, very carefully, "naturally have their fuel stored in different tanks in different places, as airplanes do. The pilots switch on one tank or another just like plane pilots. In the underground storage and fueling pits, where all the fuel for the pushpots is kept in bulk, there are different tanks too. Naturally! At the fuel pump, the attendant can draw on any of those underground tanks he chooses." The Major said curtly: "Obviously! What of it?"