We analyze successive cancellation (SC) decoder by using two random functions. The first function is related to the likelihoods of 0 and 1 in each code position, while the second gives the difference between their posterior probabilities. We then study the second power moments of both functions. We show that these moments are being squared in channel transformations, while their product tends to 0 for growing lengths $n$. This gives an elementary proof of polarization properties of SC decoding. We also derive a simple ordering of decoding channels with construction complexity of order $n\log n$.