Pinhole cameras have been around since before you were an apple in your parents eyes, since at least the 5th century BC. The image below is an example of a natural pinhole camera, something that would have caused ancient scientists and philosophers to explore the cause of the image being projected onto the wall.
The effect of pinhole cameras can be observed at this wall opposite some balistrarias in the Castelgrande of Bellinzona. You can see the red roofs of the houses and the green trees that lie behind the balistrarias being projected onto the wall.
Pinhole cameras are simply light-proof boxes with a tiny hole created on one side to project the image onto the opposite side where you could have it expose film or show through a translucent film on the back side (you may have seen something like this in grade school while viewing solar eclipses). Typically, the smaller the hole the sharper the image, but it must be a perfect circle and not too small or it increases diffraction.
Pinhole cameras are simple to make and a great way to teach your children or friends about photography. Be careful though, creating a pinhole camera can be addictive and before you know it you’ll have spend hundreds of dollars creating a large format pinhole camera.*
*Pinhole cameras can be made with objects found around the house, you don’t need to buy more than a roll of film if you don’t want to. You start to run into expensive supplies when you expose directly to something like 11×14 photo paper which can cost over $300 for 10 sheets.
For examples of pinhole photography check out the Flickr group Pinhole Photography.