‘Meaning?’ asked Xander.

‘We’re on the shore of Lake Kivu. Its depths are full of pressurised gas. Mostly carbon monoxide, which doesn’t smell, but there’s some methane down there too and that does. Both are lethal if released in large quantities.’

‘That’s all right then,’ said Xander.

‘But it could just be the volcano.’

From the look on our faces she could tell we didn’t know which volcano she was talking about.

‘Nyiragongo. It’s only twenty kilometres away. Hasn’t erupted since 2002, but when it last did it took out half the city. It’s still active.’

‘Great,’ said Xander, but I could see he was impressed with how much she knew.

Amelia looked for a moment as if she might be about to correct him, but she stopped herself and instead went off in a different direction: ‘But that’s still quite a long way for a smell to blow, I think. Actually I don’t know: how far do smells carry? A strong wind can blow for thousands of miles, but surely if it was a real gale it would disperse the smell? You don’t ever hear of tornados carrying smells. Breezes do though. Wolves and bears can smell things wafted at them from miles away, but we’re not them, we’re humans. I’ve never thought about it much before, but what’s the furthest distance a human being can smell, do you reckon?’

Amelia’s monologues make me smile. But neither of us had an answer to her question. I was staring at the green plastic fence running round the edge of the garden, thinking, or at least pretending to, when a movement caught my eye. From behind what looked like a water butt in the corner of the garden a rat ambled into view. I hate rats. This one was large and grey and in no hurry. Xander saw it at the same time as me. Without saying anything, we both leaned forward and each picked up a rock from the pile at our feet. I expected the movement might make the rat scuttle away, but it just stopped and eyeballed us. The horrible thing was less than ten metres away. I drew my hand back slowly, preparing to unleash, and Xander did the same. But I hesitated. The shot hardly seemed fair. It wasn’t until Xander threw his stone and missed, startling the rat – it jumped high in the air – that my revulsion kicked in. I aimed at the gap between the water butt and the fence and hit the rat squarely as it fled. I’d thrown a fair-sized stone. I’m not sure if you’d call it instant, but by the time the three of us had crossed the patio to have a look, the twitching was over, the rat dead.

Innocent, Patience and Caleb came into the garden at that moment.

When Caleb worked out that it was me who’d killed the rat he made a good show of being unimpressed. ‘Not really an act of conservation as such, was it?’ he said.