Note 13 at Joh 19:37: The soldiers pierced Jesus' side instead of doing the customary breaking of the legs, as they did with the two thieves, because Jesus was already dead (see note 12 at Joh 19:36). To make sure that Jesus was dead, or perhaps as a further act of brutality, they thrust a spear up through His side and into the pericardium (the sac that surrounds the heart), explaining why water came out of His side.