"One, two, three ... ."

"What are you doing?" Billy's mother crouched down beside her son so she could make eye contact with him.

Billy pointed at the starscape the lay beyond the reinforced crystal of the viewport. "I'm counting," he said with all the pride that a five-year old could muster.

"Counting what?"

"The stars."

Billy's mother laughed at the seriousness in his voice. "But there are far too many stars to count," she said and ran her fingers through her son's unruly thatch of blonde hair. Billy squirmed beneath her hand, embarrassed at his mother's treatment. He wriggled his way out of her grasp.

"Now you've made me lose count." Billy stared at the distant pinpricks of light and tried to find where he had started counting from the last time.

"Well, the ship is going to jump soon." Billy's mother stood up and held out her hand for her son. "the stars will be different and then you'll lose your place again."

"How can the stars be different?" Billy took his mother's hand and looked up at her.

"The ship is going to make a big jump across space, to our new home. That's at another star, and it's so far away that the stars you'll see are going to be different."

"How far away is that?"

Billy's mother looked down at her son's earnest face. "You know how fast light goes?"

"It's the fastest thing."

"Exactly. Well, where we're going to would take light twenty years to get from here to there."

Billy digested this information, and his eyes widened in realisation. "How long will it take for us to get there?"

"It will take twenty years for the ship to get there."

"So I'm going to be twenty-five?"

His mother shook her head. "No. You'll still be five. You see, it will take the ship twenty years to get to our new home, but it won't take any time for us. You see, when you go as fast as light, then time doesn't happen."

"Does that mean - ?" Billy's upper lip began to wobble and tears began to form in the corners of his eyes. "Does that mean that I'll miss twenty birthdays?"

Billy's mother dropped to her knees so she could take her son in her arms and hold him tight. "Don't be silly. You'll still be five. And you'll still have all those birthdays to come." She paused to work out how she could explain the concepts of time-debt and interstellar travel to her child, then decided this wasn't the time. "They'll come along with us. Don't worry."

A siren began to sound, and a voice came over the ship's PA system. "Attention! Jump will occur in one minute!"

Billy's mother turned so both she and Billy could see out of the viewport. "Now, let's watch the stars change together."

And they did.