An entrainment defect is a term used in metallurgy to describe a defect created in a casting by the folding-over of the oxidized surface layer of the molten metal into the bulk liquid. Outside of special cases, the surface of a molten metal is covered with a thin, solid layer of oxide. If the surface of the liquid happens to fold in on itself, or if a separate droplet falls into it, there will be a solid-on-solid interface between the two films. This can happen as a consequence of any kind of turbulence during the processing of the molten metal. Once solidified, this interface become functionally indistinguishable from a crack.