  The Interface Region Imaging Spectrograph (IRIS) reveals small-scale rapid
brightenings in the form of bright grains all over coronal holes and the quiet
sun. These bright grains are seen with the IRIS 1330 \AA, 1400 \AA\ and 2796
\AA\ slit-jaw filters. We combine coordinated observations with IRIS and from
the ground with the Swedish 1-m Solar Telescope (SST) which allows us to have
chromospheric (Ca II 8542 \AA, Ca II H 3968 \AA, H\alpha, and Mg II k 2796
\AA), and transition region (C II 1334 \AA, Si IV 1402) spectral imaging, and
single-wavelength Stokes maps in Fe I 6302 \AA at high spatial (0.33"),
temporal and spectral resolution. We conclude that the IRIS slit-jaw grains are
the counterpart of so-called acoustic grains, i.e., resulting from
chromospheric acoustic waves in a non-magnetic environment. We compare slit-jaw
images with spectra from the IRIS spectrograph. We conclude that the grain
intensity in the 2796 \AA\ slit-jaw filter comes from both the Mg II k core and
wings. The signal in the C II and Si IV lines is too weak to explain the
presence of grains in the 1300 and 1400 \AA\ slit-jaw images and we conclude
that the grain signal in these passbands comes mostly from the continuum. Even
though weak, the characteristic shock signatures of acoustic grains can often
be detected in IRIS C II spectra. For some grains, spectral signature can be
found in IRIS Si IV. This suggests that upward propagating acoustic waves
sometimes reach all the way up to the transition region.
