50 State Quarters to form what is known as a "mule". Only sixteen of these specimens, produced on dollar planchets, escaped from the mint. A 2005 Minnesota double die quarter, as well as a 2005 Minnesota quarter with extra trees (another die error), have both triggered numismatic interest. An unusual die break on some 2005 Kansas quarters created a humpback bison. Relatively more common are Kansas quarters bearing the motto "IN GOD WE RUST." The United States produces proof coinage in circulating base metal and, since 1992, in separately sold sets with the dimes, quarters, and half-dollars in silver. For the silver issues,
What Is Life Harrison performances" (just three of which date from the ex-Beatle's solo years) and described the track as an "exultant song of surrender", abetted by Harrison's "pumping fuzz guitar" and the song's "singalong magnetism". AllMusic's Richie Unterberger similarly praises "What Is Life" for its "anthemic" qualities, "particularly snazzy horn lines", and a guitar riff that is "one more entry in the catalog of George Harrison's book of arresting, low, descending guitar lines". Writing in the book "1,000 Recordings to Hear Before You Die", author Tom Moon refers to "the upbeat single 'What Is Life'" as an example of how Harrison "grabs