barred doors?

[Hillbilly_Dave]

I remember one of them opened for me once but I don't know how I got it to open. I've tried bombing them but that doesn't work. Any help?

[Spraks]

http://bindingofisaac.wikia.com/wiki/Challenge_Room
http://bindingofisaac.wikia.com/wiki/Boss_Challenge_Room

These rooms can only be entered if the player is at full health (or the equivalent of full health, made up with Soul Hearts). It is also possible to enter the challenge room without full health if it borders a secret room and you blow up the wall in the secret room that leads to the challenge room; however, you can't use bombs to exit the Challenge Room until you have finished it. Having the XRay Vision Item makes it possible to leave back into the Secret Room, though.

[Glass of water™]

sometimes its bosses sometimes its after the room has been defeetad