Someday, this might be important to you, too. For instance, you've been locked out of your house and someone is ransacking your valuables. There are basically three ways to get through a door without using the key or picking the lock that do not include simple brute force, like running a car into it - which might be a working solution for you, although most doors are sheltered to avoid that sort of thing. Drive right through the wall if it's important enough, it's really not much stronger than a fence in most cases.

The first way is to wind up a good stomping kick and plant your heel on it, usually right over the deadbolt. Most door frames are made of relatively thin material and furthermore are usually made of soft wood, and will readily splinter if you apply sufficient force. The deadlock bolt usually only protrudes an inch or less into the door frame. However, sometimes people have actually put some very long screws (say 4 to 5") through the lock plate and into the door frame and the stud behind it which is usually a 2x4 or 4x4.

If you can't kick it down, you're going to have to pry it open. You'll need a crowbar or a pry bar and a way to insert it, and you can either put it in between the door and the trim and pry off the trim, then pry in between the door and the frame, or you can somehow get the crowbar under the deadbolt's face on the door and pry that off, giving you access to the workings of the bolt. This is much more difficult and if you trash the bolt you might not be able to draw it out anyway.

The third and most obvious way to open a locked door only works if it swings outward - remove the hinge pins. Doors are usually mounted with two or three (likely three on an external door) hinges which are screwed in by a good three or four inches. However, the pins are usually removed with a flat-blade screwdriver and a light hammer by tapping them upwards. You can prevent someone from doing this by drilling opposing holes in your door and door frame and inserting bits of dowel into them such that they stick out far enough to lodge in the holes in the door, but not far enough to where the door becomes difficult to close.

Other than these brilliant ideas, and without involving power tools, the last option is to use something that is heavy yet easy to wield as a battering ram. Police have a device which is designed for this use, which is basically a big piece of steel pipe with a flat cap on the end and handles welded on, very similar to the device used to pound metal stakes in the ground. In the UK this is sometimes called a Glasgow Doorknocker or (the) Chubb Master Key. If you had a couple of people you could simulate this with handles made of long strips of cloth or of belts, and either some large lumber or a piece of a medium-sized tree; though if you can cut down a tree, you can probably cut down a door, or just go through a wall.

Interior doors are usually made of hollow wood. If the trick of kicking at the deadbolt position doesn't work, just kick or bash your way through the middle of the door. Some hollow doors have a reinforcement in the center, so you might need to break through 1/4 of the way from the non-hinged edge, and reach through to turn the doorknob.

If you're faced with one of those metal doors (not a metal-clad door, which is not functionally different to you for these purposes than a normal wooden door) then the only way to get in without a cutting torch or something similar (an arc welding unit will burn right through that stuff for example) is to use a prybar. Industrial metal doors usually have their locks at the top and bottom of the non-hinged edge, especially if they have a crash bar on the inside to open them.