Carrying the cool tool around paid off last night. I came upon a young cyclist in the rain, bike upside down, explaining to someone on the phone that the chain wouldn't stay on. When she finished her call (I think it was a she, but it was dark, my glasses were wet, and she was so wrapped up I couldn't be sure), I asked if I could help. She explained that she'd come off her bike and now the chain wouldn't stay on. On inspecting the single chain ring, it was bent, and deviating from true by about 1cm. The plastic chain guards either side were cracked and unable to keep the chain on the wobbly ring.
Another cyclist turned up to help. I said to the first that I might be able to straighten the ring but there was a risk it could break instead of bend. She said it was okay to try it because her folks didn't have a car so her only alternative was walking home the 12 miles. (That wouldn't have been necessary: I was only 2 miles from home and I would have cycled home and come back in the car for her and bike if I hadn't been able to fix it). She had started at college the previous week and was cycle commuting.
I got my cool tool (with adjustable spanner) out and adjusted it to the width of the ring. She held her front light so I could see what I was doing (it was properly dark by now) while the other cyclist braced the bike. It took about 5 minutes of gently 'cold setting' the ring back to a straight enough shape that the chain would stay on it. The other guy suggested changing gear on the back to get as straight a chainline as possible, and advised her to stay in that gear. I asked her if she was okay to ride and she said yes - her knee was a bit sore, but she said she could manage.
The three of us set off and the chain stayed on for the next 1.2 miles: the point at which me and the other cyclist parted ways with the YP. Before we did so I said she should get her bike checked out before her next journey, and she said her dad could do that. I hope she made it home. I think she should have done: everything was in tact and - barring another incident - it looked robust enough.
I've occasionally thought about dropping the cool tool from my armoury: I have a full-length 15mm ring spanner for the nuts on the wheels of the fixed bike, and pedal removal with the adjustable spanner would be so hard that I'm not sure it would even work. But I'm fond of it, and you never know when an adjustable spanner might come in handy - like last night.