and saw us, and he gasped.

'"Oh no!"" he said, pulling his head in and slamming the door.

"Who was that?"

Brain's Dad was frozen solid.

"That was me!" I said in a very faint voice, "I mean, that was a copy of me. Have I got a twin brother? Is that what the secret is? I've got a twin brother, and no-one's ever told me? What's going on here?!"

"No," said Brain's Dad, "He's not your twin brother."

"Who is he then?"

"He's your replica."

"My what?"

"Don't tell him Dad!" said Brain. He was out of bed, and half into a dressing gown, and he was coming down the hall towards us, unsteady on his feet. "Please don't tll them. They're my friends!"

"Its too late," said Brian's Dad, "They've seen it. I can't erase it from their memory banks without permission."

"But they'll change now," said Brian, 'They'll know what they are. We won't be humans any more!"

None of this made any sense to me so I did what any normal, rather stupid human would do. I opened the door and looked.

Inside the room there was nothing but machinery, and small pieces, and computers, and wires, and every kind of gadget you could think of. And over against one wall was a line of boys. They were exactly the same as us, plus a few others who I knew from school. I called to them but they didn't move or show any sign of having heard me.

"Who are they?" I asked, feeling dumber than ever.

"Please forgive me," said Brain.

His Dad put a hand gently on my shoulder and tried to pull me away from the door.

"We are the last humans," he said, "My Dad and me and a few others – we're the only ones left! We didn't want you to know. We thought it was better if you thought you were human too."