Put your hands palm down and try to slide one onto the other. The thumbs will not match. Some molecules have mirror twins with just the same problem.
Chemists call it chirality (kye-ral-ih-tee), or handedness. The atoms join in the same order, but their shapes face opposite ways. Your nose can spot the difference: one form of carvone smells of spearmint. Its twin smells of caraway.
Cells usually build proteins, their working parts, from left-handed amino acids. There are exceptions. Glycine has no left or right form at all, and cells use right-handed molecules for other jobs. The shared protein habit is the puzzle.
Shape matters because molecules have to fit together. An enzyme, a molecule that helps a reaction along, can fail to grip the wrong mirror shape. A mirror-built enzyme can work with mirror-built parts.
Nobody knows how life settled on its shared choice. Twisted light can tip a mix in a lab, and some reactions turn a small lead into a big one. Chance and basic physics offer other ideas. Meteorites and asteroid dust carry clues, but their mixes do not all lean the same way. The wrong hand still will not fit the glove.
