  Beneath the fusion-encrusted surfaces of the most primitive stony meteorites
lies not homogeneous rock, but a profusion of millimeter-sized igneous spheres.
These chondrules, and their centimeter-sized counterparts, the
calcium-aluminum-rich inclusions, comprise more than half of the volume
fraction of chondritic meteorites. They are the oldest creations of the solar
system. Their chemical composition matches that of the solar photosphere in all
but the most volatile of elements, reflecting their condensation from the same
pristine gas that formed the sun. In this invited editorial, we review the
nebular shock wave model of Desch and Connolly (Meteoritics and Planetary
Science 2002, 37, 183) that seeks to explain their origin. While the model
succeeds in reproducing the unique petrological signatures of chondrules, the
origin of the required shock waves in protoplanetary disks remains a mystery.
Outstanding questions are summarized, with attention paid briefly to competing
models.
