11. The Slide Rule
Before calculators and smartphones, the slide rule was a key tool for engineers, students, and scientists. This handheld device helped with mathematical calculations like multiplication and division using sliding scales. It looks simple but required skill to use accurately. Slide rules were common on desks and in classrooms until the late 1970s when electronic calculators took over. Though outdated now, they represent a clever way people solved complex problems with no batteries or screens. Finding one today might puzzle younger generations who only know digital gadgets.