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We show that simple kinematic arguments can give limits on the timescale andamplitude of variabilities in gamma-ray burst (GRB) afterglows, especially whenthe variability timescale is shorter than the observed time since the burst\Delta t < t. These limits help us to identify the sources of afterglowvariability. The afterglows of GRB 011211 and GRB 021004 marginally violatethese limits. If such violation is confirmed by the Swift satellite, a possibleexplanation is that (1) the compact objects that power GRB jets continue toeject an intermittent outflow for a very long timescale (> 1 day), (2) the GRBjet from the central engine has a temporal anisotropy with a large brightnesscontrast > 10 and small angular structure < 10^{-2}, or (3) many (> 10^{3})regions fluctuate simultaneously in the emitting site.Comment: 13 pages, 1 figure, accepted for publication in Ap

Variabilities of Gamma‐Ray Burst Afterglows: Long‐acting Engine, Anisotropic Jet, or Many Fluctuating Regions?