I just ordered a copy of 大人の科学 magazine (Otona no Kagaku, Science for Adults) Volume 25.
Each volume of this amazing magazine series from Gakken Education Publishing comes with a DIY kit to build really cool stuff, like the Berliner Gramophone from Vol.6 or the 8mm Mini-Projector from Vol.15. If I had the money I would get those two plus Vol.26 which is a Mini Electric Guitar!!
Vol.25, what I ordered contains the kit to make a Twin Lens Reflex Camera and is the third camera in the series. Vol. 3 was a simple pinhole camera, and Vol.14 had a stereo pinhole camera with three holes. First and foremost however is that with pinhole cameras or even with a stereo pinhole camera is that you can't focus them. The second is that you MUST have a steady mount for the camera. Because you must do a really long exposure time with them, any jiggling of the camera will smear the photo. Third you can never be sure just how the shot is being framed, there is no way for you to see what the film sees so there is always a good chance that the object you want to photograph is a bit to the left, or right, or is half-way cutoff as it's at the top of the frame or the bottom.
With a Twin-lens reflex camera there are two focusable lenses, one in front of the aperture and the second in front of a mirror that bounces the light up to a smoked plastic screen, allowing you to see the subject as it will be framed on the film. Because there is a shutter you can hold it in your hand when you take a picture and you can now play with the picture a bit by double- or triple-exposing the film as it's advanced by hand.
I wish I could read Japanese! ... I also wish we had a magazine like this here!
There is a flickr pool for pictures taken with the Gakkenflex:
http://www.flickr.com/groups/gakkenflex/pool/
If you want one...
I ordered mine from MAKE, which is also stocking other Gakken kits:
http://www.makershed.com/SearchResults.asp?Cat=65
MAKE has the Gakkenflex listed for $38.99US
Four Corner Store also has the Gakkenflex camera kit for sale but for $55US:
http://www.fourcornerstore.com/products/gakken-flex-35mm-tlr-kit-camera
If you do get one, please note that a few blogs have noted that there is a misprint in the instructions where the springs A and B are mislabeled. This has been corrected in the second production run.