In his arms he clutched a small black box.

"It's the parallel universe business, of course," he said to Pete, with
Tommy beaming over his shoulder. "The Grdznth can cross through. They've
been able to do it for a long time. According to our figuring, this must
involve complete control of mass, space and dimension, all three. And
time comes into one of the three--we aren't sure which."

The mathematician set the black box on the desk top and released the
lid. Like a jack-in-the-box, two small white plastic spheres popped out
and began chasing each other about in the air six inches above the box.
Presently a third sphere rose up from the box and joined the fun.

Pete watched it with his jaw sagging until his head began to spin. "No
wires?"

"_Strictly_ no wires," said Charlie glumly. "No nothing." He closed the
box with a click. "This is one of their children's toys, and
theoretically, it can't work. Among other things, it takes null-gravity
to operate."

Pete sat down, rubbing his chin. "Yes," he said. "I'm beginning to see.
They're teaching you this?"

Tommy said, "They're trying to. He's been working for weeks with their
top mathematicians, him and a dozen others. How many computers have you
burned out, Charlie?"

"Four. There's a differential factor, and we can't spot it. They have
the equations, all right. It's a matter of translating them into
constants that make sense. But we haven't cracked the differential."

"And if you do, then what?"

Charlie took a deep breath. "We'll have inter-dimensional control, a
practical, utilizable transmatter. We'll have null-gravity, which means
the greatest advance in power utilization since fire was discovered. It
might give us the opening to a concept of time travel that makes some
kind of sense. And power! If there's an energy differential of any
magnitude--" He shook his head sadly.

"We'll also know the time-differential," said Tommy hopefully, "and how
long the Grdznth gestation period will be."

"It's a fair exchange," said Charlie. "We keep them until the girls have
their babies. They teach us the ABC's of space, mass and dimension."

Pete nodded. "That is, if you can make the people put up with them for
another six months or so."

Tommy sighed. "In a word--yes. So far we've gotten nowhere at a thousand
miles an hour."

* * * * *

"I can't do it!" the cosmetician wailed, hurling himself down on a chair
and burying his face