To make whiskey, distillers create a mash, or fermented alcohol solution from a mixture of grains, yeast and water. If you've ever wondered what, besides spelling, separates American whiskey from Irish whiskey and Scottish whisky, the answer is (at least in part) the ingredients. Broadly speaking, American whiskey (also called bourbon) is usually made from corn; Irish whiskey from a blend of malted and regular barley; Scottish whiskey (Scotch) from only malted barley. After the mash is made with its respective grain, whiskey makers pour it in distillers, or special containers that boil off the methanol—alcohol that famously makes humans go blind. That leaves behind ethanol, the alcohol that we think of as, well, alcohol, along with the flavors of the original mash. The remaining liquid is put to age in charred oak barrels, which is where scotch gains it's guaiacol. Charring wood creates wood creosote, so as the liquid interacts with the barrel's walls, guaiacol migrates into the liquor.