A bicycle is surprisingly stable for an upright, two-wheeled vehicle that needs to be propped against a wall when it's not moving. But perhaps a bigger surprise is that no consensus exists on why the bike is as stable as it is.
For such a simple design, which almost anyone can understand, this seems crazy. After all, we live in a world of self-driving cars and safe passenger airplanes. Surely the bicycle can't still hold any physics or engineering mysteries?