  In the standard picture of cosmological structure formation, the Universe we
see today is evolved under the gravitational instability from tiny random
fluctuations. In this talk I discuss the onset of non-linearity in the large
scale structure formation of the Universe when the linear perturbation theory
break downs. Using 1D Zel'dovich Approximation which provides an exact solution
for density evolution, I illustrate two effects: mode spawning and mode merging
and their connection to mode coupling. Those mode couplings (quadratic, cubic
>...etc.) from gravitational clustering are in fact what the polyspectra
(bispectrum, trispectrum...etc.) are meant to measure.
