One day you woke up and clicked on a story about The Stanley Parable: HD Remix on Eurogamer.
You read that it had just arrived on Steam Greenlight and was looking for votes to get onto Valve's distribution service.
You became somewhat frustrated with the peculiar second person writing, but you stuck with it anyway because there were neat pictures below.
Or maybe you were just wondering what The Stanley Parable was in the first place.
It was at that point that the author explained to you that The Stanley Parable is a mod for Half-Life 2 in which a man named Stanley is guided through a story by a narrator, while the player can choose to obey the past tense story he's meant to enact or break from the narration, at which point the narrator would become increasingly frustrated with his misbehaviour.
The whole thing was a clever satire on game storytelling and the friction between game designers crafting an authored narrative and the player, who is free to muck about. Check out the original mod here. (Note: the game does not require Half-life 2 on PC, only the Source SDK. The Mac version however does require Half-Life 2).
After reading the above entry you checked out the latest screenshots below and told all your friends about it, because that's what you do.