If you didn't get here through The Three Doors of Serendip exhibit you may want to go there first.
!? Spoiler !?
It DOES matter if you stay or switch.
You have 2 chances in 3 of winning if you switch, and only 1 chance in 3 if you stay.
The odds of winning are twice as good if you switch than as if you stay.
If you're still unsure about this, go to Door 2.
If you're puzzled by it or would like to think more about why, go to Door 3.