  Quasars shine brightly due to the liberation of gravitational energy as
matter falls onto a supermassive black hole in the centre of a galaxy.
Variations in the radiation received from active galactic nuclei (AGN) are
studied at all wavelengths, revealing the tiny dimensions of the region and the
processes of fuelling the black hole. Some AGN are variable at optical and
shorter wavelengths, and display radio outbursts over years and decades. These
AGN often also show faster variations at radio wavelengths (intraday
variability, IDV) which have been the subject of much debate. The simplest
explanation, supported by a correlation in some sources between the optical
(intrinsic) and faster radio variations, is that the rapid radio variations are
intrinsic. However, this explanation implies physically difficult brightness
temperatures, suggesting that the variations may be due to scattering of the
incident radiation in the interstellar medium of our Galaxy. Here we present
results which show unambiguously that the variations in one extreme case are
due to interstellar scintillation. We also measure the transverse velocity of
the scattering material, revealing a surprising high velocity plasma close to
the Solar System.
