  Studying the nature of spiral arms is essential for understanding the
formation of the intricate disc structure of the Milky Way. The European Space
Agency's Gaia mission has provided revolutionary observational data that have
uncovered detailed kinematical features of stars in the Milky Way. However, so
far the nature of spiral arms continues to remain a mystery. Here we present
that the stellar kinematics traced by the classical Cepheids around the Perseus
and Outer spiral arms in the Milky Way shows strikingly different kinematical
properties from each other: the radial and azimuthal velocities of Cepheids
with respect to the Galactic centre show positive and negative correlations in
the Perseus and Outer arms, respectively. We also found that the dynamic spiral
arms commonly seen in an N-body/hydrodynamics simulation of a Milky Way-like
galaxy can naturally explain the observed kinematic trends. Furthermore, a
comparison with such a simulation suggests that the Perseus arm is being
disrupted while the Outer arm is growing. Our findings suggest that two
neighbouring spiral arms in distinct evolutionary phases - growing and
disrupting phases - coexist in the Milky Way.
