There is one thing that science has yet to explain about bicycles: how exactly they stay up. You were probably taught in school that gyroscopic force keeps those spinning wheels upright. That was thought to be true until the 1970s, when it was proved that a bicycle wheel simply doesn’t have enough mass for its gyroscopic effect to keep a rider from falling. The next theory was that bikes stay upright through the “caster effect”. Picture how the caster wheels on a shopping cart are always able to turn so that they’re pointing the opposite direction that the cart is moving. In that same way, it was thought that as a bicycle wheel begins to tip out of alignment, the caster effect would cause it to turn slightly and correct itself. But then a team of physicist invented a modified bicycle with special features that cancelled out both gyroscopic and caster effects—and sure enough, it worked just as well without them. Since then, the prevailing theory of how a bicycle works is… nothing. No one knows. It’s a phenomenon so common that a child can learn to control it, yet so mysterious that the greatest minds in the world have yet to figure it out.