Water?
I have no time to blog today, so instead I'll offer you the introductory paragraph from a Washington Post article and let you do the rest--sort of in the vein of Ron Pattinson's Protz Shield/Papazian Cup awards (which, god willing, I shall never win):
Beer’s third-most-important ingredient, after water and barley, is almost as mystifying to most beer drinkers as Prohibition. But a new kind of beer has recently emerged that gives consumers an unprecedented ability to learn: the single-hop India pale ale.Poor yeast.