  The redshifted 21cm line signal from neutral hydrogens is a promising tool to
probe the cosmic dawn and the epoch of reionization (EoR). Ongoing and future
low-frequency radio experiments are expected to detect its fluctuations,
especially through the power spectrum. In this paper, we give a physical
interpretation of the time evolution of the power spectrum of the 21cm
brightness temperature fluctuations, which can be decomposed into dark matter
density, spin temperature and neutral fraction of hydrogen fluctuations. From
the one-point statistics of the fluctuations, such as variance and skewness, we
find that the peaks and dips in the time evolution are deeply related to X-ray
heating of the intergalactic gas, which controls the spin temperature. We
suggest the skewness of the brightness temperature distribution is a key
observable to identify the onset of X-ray heating.
