The story in three lines. At fifteen, Philo Farnsworth drew a design for all-electronic television on his chemistry teacher's blackboard. At twenty-one he sent the first all-electronic picture, a single line. When RCA claimed the idea first, his old teacher reproduced the 1922 drawing from memory, and the patent office ruled for Philo.

One question to ask. "Which line would you keep?" Then: "Which memory do you trust more, the boy's or the teacher's?" A good answer notices that both are memories, and that only one was given under oath.

One thing to do at the table. Draw a simple picture on squared paper, then "send" it to someone across the table by reading out each row, square by square: dark or light. Can they rebuild it? That is what a television did, very fast.

The grown-up layer. The famous potato-field story is the inventor's own account, and nobody else was there. The decisive evidence was a teacher who stayed after school. Teenagers may enjoy hearing that a teacher's memory changed the history of television, and that a source can be important without being perfect.

Ask for the line before you offer yours.