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Watertight is a term to describe whether or not a mesh object is sully stitched closed. If the mesh has open holes in any location, it is not considered “watertight,” as if you filled that mesh with water it would leak through the holes. Its good practice to make your mesh objects watertight where appropriate, and in some cases, your shaders may actually require it (SSS typically does). If you are unsure as to whether or not your mesh is watertight, the STL format can often be used to check for watertight meshes.