A beach cruiser bicycle converted into a mountain bike, and the origins of mountain biking in general. From the early 70s, when pioneering BMXers who outgrew their 20-inch bikes started riding and modifying beach cruisers, as road bike
wheels were too narrow and
slick to work well in the dirt. Usually distinguishable by the round frame(most mountain bikes are boxy), BMX-style bars, knobbly mountain bike tires and coaster brakes instead of modern
pinch or disc brakes.
That old klunker was fun to bomb trails on when I was your
age, but these days I need either a full
suspension bike or an on-call chiropractor.