Secret Calculator is an ordinary calculator, but with a polarizing filter removed from it. The result is that you can't see anything on the display, unless you have a polarizing filter. As you turn the filter, you can make the display go from black-on-white to white-on-black.

3D movies use polarization of light to present different views to your two eyes, causing things to appear to be at different distances. Without the glasses, the movie would appear blurred.

The Secret Calculator has two polarizing filters attached to it. By overlapping the two filters and looking through them as you turn one, you get some idea what the "polarization" effect is. Looking at the calculator through one of them lets you see what number has been entered.

If you have a cheap calculator, a very sharp knife (like an Xacto), and a polarizing filter, you can make your own "secret calculator." The front surface of the calculator display is a plastic polarizer, and there is a glass layer below that: scrape off the plastic (it will probably come off in big pieces).