  Planet-planet scattering best explains the eccentricity distribution of
extrasolar giant planets. Past literature showed that the orbits of planets
evolve due to planet-planet scattering. This work studies the spin evolution of
planets in planet-planet scattering in 2-planet systems. Spin can evolve
dramatically due to spin-orbit coupling made possible by the evolving spin and
orbital precession during the planet-planet scattering phase. The main source
of torque to planet spin is the stellar torque, and the total planet-plane
torque contribution is negligible. As a consequence of the evolution of the
spin, planets can end up with significant obliquity (the angle between a
planet's own orbit normal and spin axis) like planets in our Solar System.
