Sentence:  pieces of <WSD>mother</WSD>", "adding <WSD>mother</WSD> to vinegar 
Meaning of the word marked with <WSD> tags:  A stringy, mucilaginous or film- or membrane-like substance (consisting of acetobacters) which develops in fermenting alcoholic liquids (such as wine, or cider), and turns the alcohol into acetic acid with the help of oxygen from the air.