Women's Bicycle Frames
Why are men's bikes and women's bikes shaped differently? In history, women used to wear long dresses, and they would get caught in the high frames of men's bicycles. Women's bikes were then designed with a lower frame, helping to keep dresses from tangling in the pedals.
Post-Its
Post-Its were invented in 1974 as a way for holding bookmarks. The sticky substance on the back was an accidental invention, and while they were a much-needed office supply for years, they're mostly used as pranks now.