When I was young, I was playing with a friend. Of course, the topic soon turned to mathematics (as it always does), and we got into an argument over something. We went on for some time, then he got bored of rhetoric and grabbed a Connect Four board off the games shelf.
Taking a marker from the stationery cabinet, he drew a single straight line, presented it to me. I admitted that he was right, and we moved on.
All these years later, I only have a foggy memory of the conversation. Can you work out what the argument was over, and how he made his point?
I'm pretty sure I was in Year 2/3 or so then, so use that as a guide...
This story is not by any means historically accurate