  We present here the discovery of rapid, large amplitude intraday variability
in the compact flat-spectrum radio quasar 1156+295. The detection of 40% flux
density variations at 15 GHz on a timescale of only 2.7 hours was
serendipitously made when the source was observed with the Very Long Baseline
Array as a part of the MOJAVE survey programme on February 5, 2007. Intraday
variability on timescales of a few hours or less is rare, and there exist very
few sources that show large-amplitude variations on a timescale as short as
what is now observed for 1156+295. The shape of the visibility function of the
source changes very little during the observation, although the correlated flux
density changes by 40%. This suggests that the variability occurs in a single
dominant compact component. The observed variability characteristics are
consistent with interstellar scintillation in nearby, highly turbulent medium.
The rms amplitude of modulation at 15 GHz is unusually large and it implies a
rather high scattering measure along the line-of-sight towards 1156+295.
