  With the advent of the first luminous sources at Cosmic Dawn (CD), the
redshifted 21-cm signal, from the neutral hydrogen in the Inter-Galactic Medium
(IGM), is predicted to undergo a transition from absorption to emission against
the CMB. Using simulations, we show that the redshift evolution of the sign and
the magnitude of the 21-cm bispectrum can disentangle the contributions from
Ly$\alpha$ coupling and X-ray heating of the IGM, the two most dominant
processes which drive this transition. This opens a new avenue to probe the
first luminous sources and the IGM physics at CD.
