Mr Perkins pushed his cap back and absently scratched his head as he studied the door frame. Outside, the moonlight glinted off of the snow coating the forest floor.

"Mm-hmm," he muttered ominously.

He turned to face the Carpenter family, standing in an anxious semi-circle, awaiting the builder's judgement. Behind them the August sunshine streamed through the opposite window.

"What you got here is an actual portal to a parallel universe. Was that what you were going for?"

Mr Carpenter shook his head. "No, we built a pair of French windows."

"No, you may have thought that's what you were building, but what you actually built was a portal. Did you hire professional help?"

Mr Carpenter looked at his feet. "We thought it was a simple job, so me and my brother did it."

Mr Perkins tut-tutted. "Yes, well, it's a beginner's mistake. The number of times I've seen this sort of thing. Mind you, French windows is a first. Normally it's the back of a wardrobe or an attic, sort of thing." He turned back to the portal. "Yes, you see some TV show, you nip down the builder's yard, and the next thing you know, you've got a portal into another world. You're lucky, really. It could have been a demon dimension, and then you'd be up to your ..." he glanced at the two Carpenter children, "elbows in the little devils.

"What's the thaumatic gradient?" he asked suddenly.

"Um ... I don't know," said Mr Carpenter, trying to sound as though he knew what a thaumatic gradient was.

"Let's take a rough estimate, shall we?"

Mr Perkins opened up his tool box and produced a candle. Mrs Carpenter hugged her daughter closer.

"You're not going to do anything ... you know, satanic or anything, are you?"

Mr Perkins laughed. "What, me? Do magic? Do you have any idea what the union would do to me if they found me casting spells? No, this is just a candle, relax."

He lit the candle. The flame streamed away from the window, though there was no wind. There was a purple edge to the flame.

"See that? You've got a negative thaumatic gradient." He looked at the blank expressions of the family. "You see, the thaumatic pressure on the other side of the portal is higher than on this side, so the laws of that universe are leaking into this. Sort of letting magic into the world. Have you noticed any odd incidents?"

"You mean, other than being able to see another universe through the window?" asked Mr Carpenter. He looked pensive for a moment. "Well, Peter got an A for his Maths homework this week. Trust me, that's as close to a miracle as makes no difference."

"No talking animals, plants growing monstrous, people falling asleep for a hundred years, sort of thing?" asked Mr Jones, unimpressed.

The Carpenters looked at each other. "No, not that I can think of," answered Mr Carpenter.

"Well, you're not going to be able to get planning permission, not with a negative gradient like that. Building regulations, see? Can't have a home feature that could turn you into a frog, not without a special permit. No, the thing will have to come out."

He turned back to the window and held up a tape measure across the windows. "Yeah, five foot wide ... goblin insurance ... load-bearing joist ...." He faced the family again.

"It'll cost you about seven hundred quid, give or take. I can get started on the twentieth of next month, only we got a wishing well needs repointing, then a tower to finish off."

"Couldn't we get it done any sooner?" asked Mrs Carpenter, anxiously.

"Ha, if only. I mean, the twentieth is postponing some of my existing work already. No, that's the earliest I can do it. You'll be fine till then. Just keep the doors locked and don't, whatever you do, help any little old ladies picking up tinder from the woods."

He took Mr Carpenter's elbow and turned him aside.

"These French windows ..."

"Yes?"

"Well, so long as you don't tell no-one, and if you like the idea of a portal, sort of thing, I know a wizard that could build a portal into France, if you want. Provence is nice this time of the year, I understand. Just something to think about. Real French windows, sort of thing."