Many times in our lives we take things for granted. Among them, the doors we use as we move about in residential and non-residential buildings. Doors provide several important functions, but none more important than containing fire and blocking the passage of deadly smoke and gases. Certain doors are designed to protect the means of egress; the path we must follow to reach safety in fire emergencies. Doors protect us long enough to safely evacuate.

The National Fire Protection Association (NFPA) started producing its codes and standards in the late 19th century, and NFPA continues its important work today. Over the years, NFPA produced a number of short videos that demonstrate how quickly fire can develop and spread. Some of these videos are posted on YouTube for anyone to view. Links to several of these videos are listed below, for your convenience. 

This video appears to be an excerpt of the full length video. At about 2:45 on the video timeline, the narrator points out the "breathing" of the fire that occurs near the bottom of an open door. For the purposes of this  video, there was no door leaf in the frame.

Swinging fire doors with builders hardware—as covered in Chapter 6 of NFPA 80, Standard for Fire Doors and Other Opening Protectives—are permitted to have not more than 3/4-inch clearance between the bottom of the doors and the floor surface directly below the doors when the doors are in the closed position.