Penetrant inspection, or liquid penetrant testing (PT), is a nondestructive testing (NDT) method used to find surface cracks and defects in materials like metals, ceramics, and plastics. It works by applying a colored or fluorescent liquid to the surface of the object. The liquid seeps into tiny cracks or flaws that are not visible to the naked eye. After removing the excess liquid and applying a developer, any flaws or defects become visible as the liquid trapped in the flaws shows up clearly. This method is simple, cost-effective, and widely used in industries like aerospace, automotive, and manufacturing to ensure product safety and quality.