Balsamic vinegar is made by aging a reduced grape syrup in barrels with "mother", which is a kind of bacterial slime that develops naturally in the vinegar over time. True balsamic vinegar goes through a very particular process of aging it in a series of smaller and smaller barrels, transferring it from one barrel to the next either every year, or every few years, depending on the process, and then adding fresh syrup to the largest barrel and continuing the process. The slimy film that forms on the insides of the barrels is the mother, and due to the increased concentrations over the years, the mother in the smallest barrel is most potent, and is sometimes partially removed and used to seed new batches of vinegar, so production can be expanded. Scraping the mother from the bottom of the barrel is how you multiply the goodness, the sweetness, and the quality of your balsamic.