The teacher pointed at him. “Jeremy.”

“We aren't limited to the resources of one world any more,” he answered. “We can get food and raw materials and ideas from a lot of different places, a little from here, a little from there. We don't take enough from any alternate world to hurt it.” He'd known all that stuff long before he took this class. With his mom and dad both working for Crosstime Traffic, he had to.

Ms. Mouradian knew where his folks worked. Maybe that was why she'd picked him to answer. She nodded when he was done. “That's good,” she said. “And what are some of the problems we've had since we started traveling to the alternates?”

Jeremy raised his hand one more time. He didn't want Ms. Mouradian-or anybody else-to think he didn't know there were problems. She didn't call on him again, though. She picked Michael Fujikawa instead. His folks worked for Crosstime Traffic, too. He said, “Probably contamination is the worst one.”

“That's right,” the history teacher said. “Please look at your desktops again.” Jeremy looked down. He saw the video he thought he would. There were long lines of people waiting to get shots for Hruska's disease. An early explorer had brought it back from a world that was off-limits now. There were also pictures of the blank, idiotic stares on the faces of people who'd come down with the illness. Then the desktop showed some of the plant and animal diseases and parasites that had come back here from other alternates.

A girl named Elena Ramos raised her hand. When Ms. Mouradian called on her, she said, “The other big problem is keeping people in the alternates from knowing we're visiting them.”

“Oh, yes.” The teacher nodded again. “That is the other important one. Wherever we go where there's civilization, we have to keep the secret.