Hold your hands in front of you. They are mirror images, and no amount of rotation will make them match. That property, called chirality, runs through the universe. Many molecules come in left- and right-handed forms, and living things use almost exclusively one version. Elementary particles show a related asymmetry, and the weak nuclear force treats matter and its mirror image differently.
The consequences are concrete. A drug whose mirror form is inactive may be harmless, or it may be toxic, which is why pharmaceutical chemistry cares intensely about which handedness it produces. Proteins fold into shapes that only work if their component amino acids share the same orientation. Life's preference for one hand is so consistent that it is considered a signature of biology.
Where that preference came from is unresolved. Explanations range from subtle physical effects that slightly favour one form to chance events early in Earth's history, amplified over time. The question sits at the intersection of physics, chemistry and biology, and it bears on how we would recognise life elsewhere.
