It is the first warm morning of spring at the old college, and a family of ducklings has wandered up from the river onto the freshly mown lawn of the quad. Pixel Isaac Newton steps out of the library with an apple in his pocket, and from that moment the ducklings refuse to follow anyone else. He jots a new idea in the margin of his notes: a body in motion keeps following whoever walks in front. He calls it Waddle Theory.
Every apple that drops onto the grass draws one more duckling into the line, and each new arrival makes the procession a little quicker. Honey-coloured stone walls ring the lawn, so one careless turn ends the experiment, and so does stepping on the line itself. Now and then a shiny gold coin rolls across the grass, worth a special note in the margin. How long can the line grow before the college bell rings for lunch?
