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?"

"The pushpot motors explode," said Joe. "And they shouldn't. No bomb
could be gotten into them without going off the instant they started,
and they don't blow that way. I make a guess, sir, that one of the
underground storage tanks--just one--contains doctored fuel. I'm
guessing that as separate tanks in a pushpot are filled up, one by one,
_one_ is filled from a particular underground storage tank that contains
doctored fuel. The rest will have normal fuel. And the pushpot is going
to crash when that tank, and only that tank, is used!"

Major Holt was very silent.

"You see, sir?" said Joe uneasily. "The pushpots could be fueled a
hundred times over with perfectly good fuel, and then one tank in one of
them would explode when drawn on. There'd be no pattern in the
explosions...."

Major Holt said coldly: "Of course I see! It would need only one tank of
doctored fuel to be delivered to the airfield, and it need not be used
for weeks. And there would be no trace in the wreckage, after the fire!
You are telling me there is one underground storage tank in which the
fuel is highly explosive. It is plausible. I will have it checked
immed