Schematic diagrams displaying two formation mechanisms of GSs. (a) One mature granule before the formation of a GS. The arrows in the convection zone indicate convective motions. Red (blue) color represents redshift (blueshift) signal, and the intensity of color represents the strength of the signal. This granule may undergo two evolution processes, i.e., fading and splitting (see the two cyan turn arrows). (b) to (c) The fading of the granule resulting in the formation of a GS. The blueshift signal of the granule diminishes and eventually disappears (b), subsequently the redshift signal appears, resulting in the formation of a GS (c). (d) The granule splits into two smaller granules, which separate from each other (see the green arrows). (e) A GS with a size of about 1 Mm appears between the two granules.