  In this paper, we analyze the dynamics of two layers of immiscible, inviscid,
incompressible, and irrotational fluids through a full nonlinear system. Our
goal is to establish a virial theorem and prove the polynomial growth of slope
and curvature of the interface over time when the fluid below is no denser than
the one above. These phenomena, known as Rayleigh-Taylor instability and
Kelvin-Helmholtz instability, will be proved for a broad class of regular
initial data, including the case of 2D overlapping interface.
