We naturally see in 3-D because each of our eyes sees the scene in front of us from a slightly different angle. If you take a picture of a subject, and then move the camera a little to the side and take another picture, you have captured a stereo pair of images. If you show the correct image to the correct eye, your brain will interpret it as a 3-D scene.
There are special glasses and projectors and other contraptions that allow you to view stereo pairs of images, but they really aren't necessary. Once I learned this, I started taking stereo pairs of pictures everywhere I went. Why not? It's almost as easy to take two picture as it is to take one.
The pictures in this gallery are all arranged for crossed-eye viewing. In order to see them in 3-D, focus on the screen in the middle of the two side-by-side images. Slowly cross your eyes until the two images overlap. It may help to place your finger in between your eyes and the screen and focus on that to get the images to overlap. It is important to keep your head straight (don't tilt side to side). It may be very helpful to reduce the size of your browser window so that you don't have to cross your eyes as far. Once the images are overlapped, hold it until your brain interprets it as a 3-D scene. Trust me, it will happen. When you're first learning to do this you may have to hold it for several seconds for this transformation to occur. With practice it will become almost instantaneous.