Titan's tidal deformation
Titan orbits Saturn on an elliptical path once every 16 days. The shape of the moon changes along its orbit because of the varying tidal pull from Saturn such that it is stretched into a rugby-ball shape at its closest point to the gas giant and is more spherical at its most distant point.
The change in shape causes a redistribution of mass in the moon and therefore a change in the gravity field, which is measured by Cassini in the form of the change in acceleration of the spacecraft as tracked by microwave radio links with the ground antennas of NASA's Deep Space Network.
The measurements were taken along different points of Titan's orbit and correspond to a tidal deformation of about 10 m. Since the amplitude of solid tides depends on the overall rigidity of the body's interior and on the strength of the gravitational pull of nearby objects, Titan must have a liquid ocean at depth to accommodate this amount of tidal deformation.