  We report on the changing-look nature of the active galactic nucleus (AGN) in
the galaxy NGC 4156, as serendipitously discovered thanks to data acquired in
2019 at the Telescopio Nazionale Galileo (TNG) during a students' observing
programme. Previous optical spectra had never shown any signatures of
broad-line emission, and evidence of the AGN had come only from X-ray
observations, being the optical narrow-line flux ratios unable to unambiguously
denote this galaxy as a Seyfert. Our 2019 TNG data unexpectedly revealed the
appearance of broad-line components in both the H$\alpha$ and H$\beta$
profiles, along with a rise of the continuum, thus implying a changing-look AGN
transitioning from a type 2 (no broad-line emission) towards a (nearly) type 1.
The broad-line emission has then been confirmed by our 2022 follow-up
observations, whereas the rising continuum has no longer been detected, which
hints at a further evolution backwards to a (nearly) type 2. The presence of
broad-line components also allowed us to obtain the first single-epoch estimate
of the black hole mass (log(MBH/Msun) $\sim$ 8.1) in this source. The observed
spectral variability might be the result of a change in the accretion activity
of NGC 4156, although variable absorption cannot be completely excluded.
