plained. "But really
only for convenience. Now do you want the ideal ecological differential
or don't you? Because--" Here he flourished the red ribbon again, peered
into the helmet, looked narrowly at Martin, and shook his head.

"I'm sorry," the robot said. "I'm afraid this won't work. Your head's
too small. Not enough brain-room, I suppose. This helmet's for an eight
and a half head, and yours is much too--"

"My head _is_ eight and a half," Martin protested with dignity.

"Can't be," the robot said cunningly. "If it were, the helmet would fit,
and it doesn't. Too big."

"It does fit," Martin said.

"That's the trouble with arguing with pre-robot species," ENIAC said, as
to himself. "Low, brutish, unreasoning. No wonder, when their heads are
so small. Now Mr. Martin--" He spoke as though to a small, stupid,
stubborn child. "Try to understand. This helmet's size eight and a half.
Your head is unfortunately so very small that the helmet wouldn't fit--"

"Blast it!" cried the infuriated Martin, caution quite lost between
Scotch and annoyance. "It does fit! Look here!" Recklessly he snatched
the helmet and clapped it firmly on his head. "It fits perfectly!"

"I erred," the robot acknowledged, with such a gleam in his eye that
Martin, suddenly conscious of his rashness, jerked the helmet from his
head and dropped it on the desk. ENIAC quietly picked it up and put it
back into his sack, stuffing the red ribbon in after it with rapid
motions. Martin watched, baffled, until ENIAC had finished, gathered
together the mouth of the sack, swung it on his shoulder again, and
turned toward the door.

"Good-bye," the robot said. "And thank you."

"For what?" Martin demanded.

"For your cooperation," the robot said.

"I won't cooperate," Martin told him flatly. "It's no use. Whatever fool
treatment it is you're selling, I'm not going to--"

"Oh, you've already had the ecology treatment," ENIAC replied blandly.
"I'll be back tonight to renew the charge. It lasts only twelve hours."

"_What!_"

ENIAC moved his forefingers outward from the corners of his mouth,
sketching a polite smile. Then he stepped through the door and closed it
behind him.

Martin made a faint squealing sound, like a stuck but gagged pig.

_Something was happening inside his head._

II

Nicholas Martin felt like a man suddenly thrust under an ice-cold
shower. No, not cold--steaming hot. Perfumed, too. The wind that blew in
from the op