A chronic smoker's airway biopsy shows squamous cells where columnar stood. This is metaplasia — smoke's chronic irritation reprogrammed the bronchial epithelium from columnar to hardier squamous. Adaptive, but the price: squamous cell carcinoma arises from metaplastic ground (like Barrett's esophagus → adenocarcinoma). Metaplasia is reversible if the irritant leaves — quit smoking and cilia can return. The biopsy is a warning written in cell type.