As the author of a yogurt cookbook, I should know something about skyr (pronounced skeer). Sales are growing in the U.S. for this Icelandic dairy product, but when a friend asked me how it differed from yogurt, I couldn’t say. It’s thick, creamy, tangy cultured milk—like yogurt. It’s fermented with bacteria—like yogurt. So why do Icelanders insist that it isn’t yogurt?