It was a Thursday. I know because it was Yerin's turn to scan the plants with the Geiger counter. This is one of our most important chores - we think we've moved far enough from the city, but with radiation you can never be too sure.   When Yerin finished with the plants, she remembered that it had been a while since we had checked the radio. She had to dig it out from underneath the loveseat - someone had put it there to keep it out of the way. Besides, there's nothing sadder than a radio that never picks up a signal, and might never again. Still, we check it from time to time - we can't be the only survivors. At least, I hope we can't.   Anyway, Yerin pulls the radio out from under the loveseat and hands it to me. Somehow the others are convinced that I have some sort of radio skills that they don't have. It makes them feel better, so I let them believe it - they'd be so disappointed to learn that all I do is randomly fiddle with the knobs, and occasionally shake the box if that doesn't seem to be changing anything. I almost had the others convinced we could put the radio away and go back to doing nothing when the radio picked up a hum. Not a hum like "I'm humming a song," but a hum like a machine droning away at the same note. We all got excited about this, especially when we noticed that the hum was always strongest when the radio was facing west.   Yuju was the one who came up with the plan: We'd get on the next westbound train, ride until the radio showed we had passed whatever was making the hum, then start walking back toward the house, hopefully finding the hum along the way. We checked the clock on the wall - we had just enough time to walk to the train station before the afternoon train. Quickly we grabbed the radio and the Geiger counter and whatever else we thought we might need and headed to the station.   We made it to the station with a couple of minutes to spare. Eunha was the only one who bothered getting a ticket from the machine. All of us know there are no conductors, but it makes Eunha feel better to have a ticket. We all wave at the train when it shows up, but we don't look closely enough at the windows to see that there's no engineer to wave back at us. We're glad that the robots keep the trains running, but if you think too much about it an empty train that's kept running by robots just in case someone decides to ride it is really too depressing to think about.   Once we're all in the carriage, the train stars moving again. As usual we form into pairs. This time it's Sowon and Yuju, Umji and Eunha, Yerin and me. I don't know why we always form pairs. I mean, I know we split into smaller groups because 6 people is too many for a comfortable conversation on a train - the people on the ends have trouble hearing each other.  But I don't know why we always split into three pairs instead of two groups of three.  Sowon says it's biology, that the pair-bonding instinct is a natural part of human nature. She could be right: She's 3 years older than me, so she got to go to school longer before the Incident. I'm the maknae of our group, so if there's someone who doesn't know something, it's almost always me. Eunha looks like the maknae, but that's just because she's short and has a baby face. She's actually the third oldest.   We all fall asleep on the train, and when we wake up, the radio says we've passed whatever's making the hum, so we get off the train and start walking back. Yerin says she has a secret to tell me, so we let the others get a little bit ahead of us and then she whispers it in my ear. No, I won't tell you what she said - it's a secret. Besides, it doesn't matter unless we find some boys out here and so far we