The Trundle Trial

There is an argument in your school. It is three years old, and it is about the corridor. The long corridor — the one outside your classroom. The one you are not allowed to run in, which is why everyone runs in it. How long is it? The sports teacher says seventy strides, and his strides, he says, never lie. The head girl measured it heel-to-toe on sports day: one hundred and ninety of her feet. The caretaker did it with his broom — forty-two brooms and a bit — and he trusts that broom more than he trusts anybody's feet. Seventy. One hundred and ninety. Forty-two and a bit. Same corridor. Three answers. Three years of arguing. And now it matters. The principal has ordered a red carpet for Annual Day, and the carpet shop wants one number. Too short, and the chief guest walks the last stretch on bare floor. Too long, and the carpet keeps going up the stairs. Someone tried the maths teacher's measuring tape. It is about as long as your arm. They gave up around tape-length two hundred, when the lunch bell rang and a football rolled through their chalk marks. Feet don't work — everyone's feet are different. Brooms don't work — there is one broom, and the caretaker needs it. The tape works, but life is too short. You need a machine that walks the corridor and counts its own steps. A machine whose step is always — always — exactly the same. A machine that cannot lose count, because it shouts every step out loud. Click. Today, your team is going to build one.