Seren wrote that down too. ‘Why are we doing all this?’

‘Always study the artefacts of the enemy.’ The Crow sniffed the Box and then touched it with the very edge of a feather.

‘It’s not going to explode.’

‘You never know with Them,’ the Crow said darkly. ‘Thermometer!’

It took a while to find one; Seren had to run down to Gwyn in the hot-house and borrow one off the wall, and then hurry back with it.

‘It’s probably just the same temperature as the room,’ Tomos said as they watched the red line of mercury fall and steady.

‘It’s a lot colder. And that,’ the Crow put on his best lecturing voice, ‘would suggest a supernatural, even sinister, component within that might…’

Seren frowned. She was fed up of waiting. ‘But I want you to read the words!’ She reached over the Crow’s shoulder. Then she gave a great cry of surprise. ‘It’s gone! How is that possible?’

She was amazed. There was no beautiful black swan gazing out at her from the lid. Instead the picture showed a dark lakeside, the deep water seeming to ripple on the shore, the trees shadows with stars caught in their topmost branches. Beyond, barely visible, were the turrets of an enormous castle with one lighted window in the highest tower.

In the grass sat a hare and a tiny mouse. High in a tree perched a white owl.

‘It’s a completely different picture!’

‘Hmmm.’ The Crow pushed imaginary glasses up its beak. ‘I didn’t see anything else.’

‘Neither did I,’ said Tomos. ‘It’s always looked like that.’

‘No, it hasn’t!’

‘What was there before?’

‘A black swan. Wearing a collar.’

The Crow blinked. It stared at her, astonished. ‘A collar? A diamond collar?’

‘Yes. And the words…’

Tomos said, ‘There are no words.’

Seren took a breath. She picked up the Box and stared. ‘Yes, there are. I don’t know why you can’t see them. Perhaps it’s magic. I’ll read out what it says.’

The Crow nodded, doubtful. ‘As long as you’re not making this up…’

‘I’m not!’ So she read the words out loud.

The Box of the Midnight Swan

If you can open My closed lid

Your heart’s desire Inside is hid

‘Now can you see why I bought it for you?’

The Crow didn’t answer. Its beak was hanging open and its eyes were wide with sudden dismay.

‘No!’ it whispered. ‘No. It can’t be…’

‘What?’ Seren said.

The Crow couldn’t take its eyes off the Box. It began talking very fast. ‘Right. Yes. Very well. I think we should just stop this now and go back to Latin. Maths, even. Playtime! Yes, let’s have playtime. We can get out the jigsaw puzzles and the chessboard. Table skittles…’

‘What on earth are you gabbling about?’ Seren said. She had never seen the Crow in such a panic. ‘It’s not dangerous. I can’t even get it to open… See…’

She showed them how the lid refused to move. Tomos tried too. He was baffled. ‘It’s as if it’s locked,’ he muttered, irritated. He picked the Box up and turned it over. ‘Something’s stuck on the bottom.’