McDonald's is giving away free coffee to customers who know when to go and how to ask for it. The trick (at least in Chicago) is to go on Mondays and specifically ask for a free small coffee.
The promotion isn't advertised and instead is being covered by local news sources. And because it's inside information, people are telling their friends, family and co-workers the technique to get free coffee.
The Lesson: A good secret can be a great conversation starter.