  Theoretical predictions for the ensemble quasar structure function are tested
using multi-epoch observations of Stripe 82 collected by the Sloan Digital Sky
Survey. We reanalyze the entire available volume of the g-band imaging data
using difference image photometry and build high quality light curves for 7562
spectroscopically confirmed quasars. Our structure function includes ~4.8x10^6
pairs of measurements and covers a wide range of time lags between 3 days and
6.9 years in the quasar rest frame. A broken power-law fit to this data shows
the presence of two slopes alpha_1=0.33 and alpha_2=0.79 with the break at ~42
days. The structure function compiled using only flux increases is slightly
lower than that for variations of the opposite sign, revealing a slight
asymmetry between the leading and trailing edge of a typical flare. The reality
of these features is confirmed with monte-carlo simulations. We give simple
interpretation of the results in the frames of existing theoretical models.
