Monty Hall

[andrewducker]

[Poll #1770413]

Explanation

I have known what the answer was for ages, but for some reason it only "clicked" in my head today. You can blame sarahs_muse for triggering it.

Comments

[andrewducker]

The link is part of the wikipedia article in the post.

For how I internalised it, see my response to dpolicar below.