You participate in a game show where you must choose between 3 doors. Behind 2 doors is a goat, but behind 1 door is a car. You pick a door, say door no. 1, but before you open the door, the host opens another door, say door no. 3, which has the car behind it. "Oops," says the host.
You lose.
Yeah, but—and I say this having read the comments and knowing people aren't getting it—shouldn't you assume that, unless you're a moron? You thought they set up a game show so there was a chance they reveal the big prize and then stand there sheepishly going "so... uh.. do you want to pick a new door, or...?" Why would that ever be part of the design?
Also, as u/Kienan pointed out, even if it was random, you'd still switch.
I will direct to myself for the argument that switching doors when the host has no knowledge and just got lucky is meaningless. It's only smarter to switch doors if the host knowingly makes the correct choice.
It depends on how much you see the problem as a contrivance and how much as an actual scenario. Either way, the point is not to see it as a one-off event, but to run the simulation many times in order for the statistics to manifest. In every iteration of the simulation, the host must select a non-prize door.