This year has seen another batch of record A-level results, continuing a decade-long trend. It is often assumed that the exams must be getting easier, but a perusal of my daughters’ A-level papers makes me doubt this: they were really hard! Could it be that people are learning faster and better?

I think we’re in an era of generosity—a sentiment which applies both to money (witness the recent commitment of 40 US billionaires to give away at least half their fortunes) but also to knowledge. Knowledge used to be power, and as such was closely guarded. Now reputation is power—and giving your knowledge away is the best thing you can do for the benefit of your reputation.

Salman Khan was an investment banker—a bright young man (MIT, Harvard Business School) who discovered a facility for explaining mathematics, physics, history, biology and finance. It started when a cousin in Dubai asked