Before opening the door you should CAREFULLY touch the door to feel if there is any heat behind it. If there is then you get out the closest window or door away from that door as there will most likely be a fire behind it.
If the door is not hot then you can go out of it. Either way you need to get out of the building ASAP to prevent you and any others from being injured / burned.
<em>If the door is completely cool, open it cautiously. Put your shoulder against the door and open it slowly. A fire that has died down for lack of oxygen can flare up when a supply of fresh air rushes through the door you open. If heat or smoke enters the room, slam the door and make sure it is closed securely.</em>
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You should crawl to exit because you'll breathe less smoke if you stay close to the ground. Smoke naturally rises, so if there is smoke while you're using your escape route, staying low means you can crawl under most of it. You can drop to the floor and crawl on your hands and knees below the smoke.
True, it will stretch your neck, spine and chest. Also if you look up "bridge pose" and try it yourself you can actually figure out what it stretches if you wanted :)
