Most of the calculators I own have nearly as many buttons as your average nuclear submarine. Not this one: the Casio SL-800 is a basic four-function calculator. Its interesting claim of fame is that this is one of the few calculators that are truly credit card size.

It is easy to make a calculator that has the same width and height as a credit card. In fact, some of the cheapest promotional calculators that are given away as freebies are of this size. The Casio SL-800, however, is credit-card size in all three dimensions: in other words, in addition to the width and height, it has the same thickness as those ubiquitous pieces of plastic.

I received this calculator from a former employer in Austria (thank you, Harald!) back in 1986. I've been carrying it in my wallet for the next 12 years or so, I sat on it many times, it's been left in the sun, in freezing cold, and more. Yet, despite the signs of heavy use, it still works and its display remains, if less than perfect, still entirely legible.

The calculator has no batteries, only a solar cell as its power source. In fact, the entire unit is a sealed plastic card, so it's actually the only waterproof, washable calculator in my possession.

Considering the engineering compromises that had to be made to pack the electronics of a calculator into a package that's only 1 mm thick, I am quite surprised, but very pleased, to see that it still works. I no longer carry this unit in my pocket; it is now part of my cherished collection.