line on
that impostor."

Ames departed to begin a thorough check of all personnel at the plant
who might have been implicated. Bud went on an errand, as Tom began
showing his father the accomplishments of the space robot.

"We've christened him Exman," Tom said.

By means of the electronic brain, he made the visitor do a number of
maneuvers in response to orders.

"Wonderful!" Mr. Swift exclaimed, greatly impressed. "Let's see if he
can use his caterpillar treads as well as he does the wheels."

Tom brought a small flight of portable aluminum stairs which he used for
reaching up on high shelves or tinkering with outsized machines. Tom was
uncertain at first how to code the command, having no symbol for steps
or stairs. Finally he moved Exman to the bottom of the steps and
signaled simply: _Go up!_

Exman paused for a moment, then attempted the ascent. His caterpillar
tracks clawed their way up the first step. Then, gingerly, he essayed
the next. The robot body tilted, but its gyro kept it from toppling
over.

"Bravo!" Mr. Swift applauded encouragingly. But the next instant Exman
gave up! He slid back to the floor again with a heavy bump. Then he
began whirling and darting about madly.

"Good night! Exman's gone berserk!" Tom cried.

Now wafts of smoke could be seen issuing from the robot's wheels. He was
banging wildly about the laboratory, leaving a trail of havoc.

Bud, who had returned, opened the door to come in. Instantly Exman
lunged toward him, antennas sparking fiercely and wheels smoking. Bud
slammed the door hastily.

The Swifts, too, found it wiser to take cover. They crouched behind a
lab workbench until the frenzy was over. Presently Exman subsided and
rolled to a complete standstill.

"Good grief!" Tom stood up cautiously and eyed the creature. It made no
further move. Bud poked his head through the doorway for a wary look,
then re-entered the laboratory.

"What made him blow his top?" Bud asked.

Then Tom heard a quiet chuckle from his father. "Actually, boys," the
elder scientist said, "I think we should be encouraged."

"_Encouraged?_" Tom stared at his father.

[Illustration
(Tom Jr. and Tom Sr. watch as Exman tries to climb stairs)]

Mr. Swift nodded. "Yes, the whole thing was rather a noteworthy
reaction. I believe Exman was displaying a fear complex about navigating
up those stairs."

Tom gasped, then broke out laughing. "Dad, you're right! I'll bet when
its body tilted