  A model of wavelength-dependent lags in optical continuum variability of AGNs
is proposed which avoids the problems of the popular ``lamp-post'' model.
Rather than being due to reprocessing of high-energy radiation from a
hypothetical source above the accretion disk, the wavelength-dependent delays
observed from the B to I bands are instead due to contamination of an
intrinsically coherently variable continuum with the Wien tail of the thermal
emission from the hot dust in the surrounding torus. The new model correctly
gives the size, wavelength dependence, and luminosity dependence of the lags,
and quantitatively predicts observed color hysteresis. The model also explains
how the measured delays vary with epoch of observation. There must also be
contamination by scattered light and this can be detected by a lag in the
polarized flux.
