They explained it all to us when we were ten years old; I guess they thought we were too young to understand before then, although we knew a lot of it already. They told us just after we landed on Mars.
"You're home, children," the Head Teacher told us after we had gone into the glassite dome they'd built for us there. And he told us there'd be a special lecture for us that evening, an important one that we must all attend.
And that evening he told us the whole story and the whys and wherefores. He stood up before us. He had to wear a heated space suit and helmet, of course, because the temperature in the dome was comfortable for us but already freezing cold for him and the air was already too thin for him to breathe. His voice came to us by radio from inside his helmet.
"Children," he said, "you are home. This is Mars, the planet on which you will spend the r