The biology class at School 24 keeps a field notebook on the window side of the yard. Last year it held one long study: a single sunflower, measured every Monday morning for a whole term.
At the start the plant was 32 centimetres tall. After that the numbers came in a straight line: 41, then 50, then 59, then 68. Nine centimetres, every week, without one exception. Two pupils checked the tape measure twice because the sequence looked too tidy to be real.
The interesting question came from the back of the room. The old wooden fence along the yard is 122 centimetres high. When would the sunflower look over it?
The class did not guess. They reasoned. The plant had to climb 90 centimetres to get from 32 to the top of the fence, and it climbed 9 centimetres each week at a constant rate. So it needed exactly ten weeks.
They wrote the date on the wall in chalk, and on that Monday in week ten the flower stood exactly level with the top board. A week later it was above it, and by the end of the term it had passed the drainpipe.
Their teacher wrote one line under the last measurement: Three numbers were enough — where it began, how much it gained, and where it had to get to.