I'm writing a MG fantasy about a boy who goes back in time to early medieval Wales. In his first adventure, he makes some use of modern-day scientific knowledge to solve a problem at a castle, in a way that seems supernatural to the people there. I'm trying to work out exactly what piece of information he makes use of. I know the following:

1. It should be something "general-knowledge", since he's much better at history (particularly medieval history) and English than science or math. (He's not bad at the latter two, just average.) He's also around twelve, which would make it "general-knowledge" for that age. (It should also be something he would know without needing to look it up, since he didn't bring any books or other written material back with him - just the clothes he's wearing, a digital wristwatch, and a flashlight.)

2. Ideally, the problem he solves would block his journey as well as creating trouble for the people living in the castle; he's trying to get to a "time-portal" to return to his own time. (To give some idea of what I mean, the original idea I'd had was that a strange water-beast - based on the addanc, a mythical Welsh water-monster - was causing a flood that rendered a stream or river between the castle he was at and the "time-portal" impassable, as well as threatening to inundate the area around the castle, and the MC would have to get it to leave. That idea ran into problems, so I decided to replace it with something more mundane.) Thus he has two objectives in solving it - first, remove the obstacle to his journey, and second, help the people in the castle.