  We present a detailed single-pulse analysis for PSR B1929+10 based on
observations with the Five-hundred-meter Aperture Spherical radio Telescope
(FAST). The main pulse and interpulse are found to be modulated with a
periodicity of $\sim12$ times the pulsar's rotational period ($P$). The
$\sim12P$ modulation is confirmed as a periodic amplitude modulation instead of
systematic drifting. The periodic amplitude modulation in the IP is found to be
anti-correlated with that in the weak preceding component of the MP (MP_I), but
correlated with that in the first two components of the MP (MP_II), which
implies that the modulation patterns in the IP and the MP are phase-locked.
What is more interesting is that the modulation in MP_II is delayed that in the
IP by about 1P. Furthermore, high sensitivity observations by FAST reveal that
weak emission exists between the MP and the IP. In addition, we confirm that
the separation between the IP and the MP is independent of radio frequency. The
above results are a conundrum for pulsar theories and cannot be satisfactorily
explained by the current pulsar models. Therefore, our results observed with
FAST provide an opportunity to probe the structure of pulsar emission and the
neutron star's magnetosphere.
