ime machine.

"See? Five minutes before I shall place it there, it _is_ there!"

His other colleague frowned at the cube. "But," he said, "what if, now
that it has already appeared five minutes before you place it there, you
should change your mind about doing so and _not_ place it there at three
o'clock? Wouldn't there be a paradox of some sort involved?"

"An interesting idea," Professor Johnson said. "I had not thought of it,
and it will be interesting to try. Very well, I shall _not_ ..."

There was no paradox at all. The cube remained.

But the entire rest of the Universe, professors and all, vanished.

Sentry

He was wet and muddy and hungry and cold, and he was fifty thousand
light-years from home.

A strange blue sun gave light and the gravity, twice what he was used
to, made every movement difficult.

But in tens of thousands of years this part of war hadn't changed. The
flyboys were fine with their sleek spaceships and their fancy weapons.
When the chips are down, though, it was still the foot soldier, the
infantry, that had to take the ground and hold it, foot by bloody foot.
Like this damned planet of a star he'd never heard of until they'd
landed him there. And now it was sacred ground because the aliens were
there too. _The_ aliens, the only other intelligent race in the
Galaxy ... cruel, hideous and repulsive monsters.

[Illustration]

Contact had been made with them near the center of the Galaxy, after the
slow, difficult colonization of a dozen thousand planets; and it had
been war at sight; they'd shot without even trying to negotiate, or to
make peace.

Now, planet by bitter planet, it was being fought out.

He was wet and muddy and hungry and cold, and the day was raw with a
high wind that hurt his eyes. But the aliens were trying to infiltrate
and every sentry post was vital.

He stayed alert, gun ready. Fifty thousand light-years from home,
fighting on a strange world and wondering if he'd ever live to see home
again.

And then he saw one of them crawling toward him. He drew a bead and
fired. The alien made that strange horrible sound they all make, then
lay still.

He shuddered at the sound and sight of the alien lying there. One ought
to be able to get used to them after a while, but he'd never been able
to. Such repulsive creatures they were, with only two arms and two legs,
ghastly white skins and no scales.

--FREDR