I guess I can do one of these a day or so. I've collected some novelties over the years.

First up is a Curta mechanical calculator. Before electronic calculators became a thing, these were the best portable calculators in the world. Notably, they were the calculator of choice in rally car sports.

They work by a series of helical gears that act as registers. A series of internal gears and value assignment switches apply an adjustable number of incrementations to those gears, multiplying gears and the tracking gears, once per "grind." The result is output as a number on top of the device. The "clear register" function is lifting the top ring, which releases the reverse lockout on the gears and a clockwise turn on the ring then resets them to their zero state.

They were designed by Curtz Herzstark, partly before WWII and partly while he was imprisoned in a Nazi concentration camp. He had filed a patent for it in 1938, shortly before his family's manufacturey became a weapons factory. During his imprisonment, in addition to nearly starving to death, he completed his plans for manufacturing of his calculator.

It had fun names like the, "pepper grinder," and "math grenade."