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These doors are grey and usually (always?) open into a "weapon" room, i.e. there is a weapon on the minimap for them. I've gotten in two, but it was very non-obvious what I did to open the doors. For other doors, I've killed every mob on the floor, put out every fire, and dropped a bomb on the door and it still didn't open. Does anybody know what the rule is for these things?

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    I've gotten in more doors since posting this question. Some seem to trigger when I kill all the mobs in the room while others when I pickup all the items, however, there are some that just don't ever open. I'm still at a loss as to why some open and some don't...
    – Morinar
    Commented Sep 30, 2011 at 17:24

4 Answers 4

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The doors are tied to your health. The door will only be open if you have full health. However, the temporary HP boosts (blue hearts) can serve as a equivalent of the missing red hearts.

For example: If you're missing one red heart but have one blue heart the door will open. But if you're missing two red hearts and only have one blue heart the door will still be locked.

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  • Does it require "full" health, or just 3 hearts of health?
    – Chris Rasys
    Commented Oct 4, 2011 at 17:47
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    @ChrisRasys Full health - You need to have all the heart containers filled up, or the equivalent of the missing heart containers in blue hearts.
    – mordi2k
    Commented Oct 4, 2011 at 17:56
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    This answer is incomplete and covers only half of the question. See my answer (originally an edit - but rejected on the grounds of "not being an answer") for more details.
    – 7heo.tk
    Commented May 1, 2017 at 12:48
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The doors of Challenge Rooms are tied to your health:

The normal doors will only open if the sum of your current HP (counting red, blue, and black hearts) is at least equal to the number of red-hearts-containers you have.

The boss doors (recognizable from the skull they have on the door head) require the player to have only one red heart at most, but the amount of blue and black hearts are not taken into account. It is best to enter them with a Full Health pill, or with plenty of blue/black hearts (the health gambling machines can be used to reduce the amount of filled red-heart containers while keeping the blue and black hearts intact).

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If you get items such as Transcendence or Lord of the Pit, or the Hanged Man tarot card, you can actually fly through/over the door, regardless of your health level.

EDIT: I believe this applied to the original Flash game; it does not apply to Rebirth and newer. Hence my answer is valid given the time at which the question was asked.

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  • I cannot downvote this answer because I do not have enough points, but this answer is about Curse Rooms while the question is about Challenge Rooms. It is therefore incorrect. Do not waste your Hanged Man tarot cards on closed Challenge Rooms, you will not be able to enter that way. See the most voted answer instead.
    – 7heo.tk
    Commented May 1, 2017 at 11:02
  • @7heo.tk My answer was valid given the context of the question, that being the original Flash-based game, which was glitched and allowed flying through the door regardless of health.
    – Grant Palin
    Commented May 2, 2017 at 21:50
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    @GrantPalin I have the original, and no you cannot fly through these doors.
    – OrangeDog
    Commented May 3, 2017 at 12:52
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Maybe the doors didn't open because they were secret rooms. Some rooms can only be seen with x-ray glasses. That could be it.

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    That doesn't make any sense. As the accepted answer says, the door letting you in depends only on your health.
    – murgatroid99
    Commented Jan 5, 2013 at 18:46

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