If you attempt to open an emergency exit door while the plane is at cruising altitude, you will find it physically impossible. Most aircraft doors are "plug-type," meaning they are wider than the opening and are held shut by the immense difference in air pressure between the cabin and the outside; at 35,000 feet, several tons of force are pushing that door into its frame. However, if the plane is on the ground or at a very low altitude where pressure has equalized, opening the door will cause the emergency slide to deploy instantly (unless the door has been "disarmed" by the crew). On the ground, this is a major safety hazard that can injure ground crew and will result in an immediate flight cancellation, a massive fine (often $20,000+), and likely a lifetime ban from the airline. If a door were somehow compromised in flight (e.g., structural failure), it would result in explosive decompression, where loose objects and unsecured passengers could be sucked toward the opening, and the cabin would fill with a cold mist as oxygen levels plummet.