1972: HP-35 handheld scientific calculator
In January 1972, engineers put down their slide rules, mesmerized by the glowing red LEDs of the world's first pocket scientific calculator. Unlike the simple handheld calculators introduced the year before, the $395 HP-35 (so called for its 35 keys) could do trig and logs and cosines in the palm of your hand, like a true portable computer.
To spice things up, it used reverse Polish notation, which stacks math operators at the end of the equation: 6/3 was too obvious; 6 3 / was much more fun. And to yuck it up more, engineers dreamed up calculations that resulted in .07734 -- which looked like hello. when you held your calculator upside down.