Comment by TacticalCoder

We had HP ones at school, lots of fun in math classes...

But then I still have my Casio FX-850P, which I probably own since 1989 or something. Last time I put batteries in it (5 years ago?) it was still working. It's in TFA : )

https://en.wikipedia.org/wiki/Casio_FX-850P

It's on my desk, always visible. Next to an Atari Portfolio (the same one young John Connor uses to hack doors in Terminator 2) and a totally beaten up ZX Spectrum. Remnants of a glorious past.