Platelets stop the blood at injury site by clotting it. How?

Blood platelets or thrombocytes are the cell fragments produced by megakaryocytes. During any injury, platelets release a substance called thromboplastin. This thromboplastin helps in the formation of a complex enzyme thrombokinase which converts prothrombin into active thrombin. This thrombin converts soluble fibrinogen into insoluble fibrin. This fibrin forms a network at the site of injury which traps the blood & form clot.