Hybridisation

I have begun to read up on some basic organic chemistry. Currently still trying to firm up the concept of orbital hybridisation.

The concept is simple. With respect to carbon, an electron in the doubly-filled 2s orbital is promoted to the empty 2p orbital forming 4 singly-filled orbitals. This state is known as the higher energy electonic state of carbon. The 4 singly filled orbitals then hybridise in different combinations to form hybrid orbitals. It is hybridisation that allows carbon to form 4 bonds per molecule.

Satisfied with my understanding, I ran with Colin today. We were talking about chemistry and normality and stuff. Then he asked an excellent question. He wondered where the energy for hybridisation (namely the initial promotion of the 2s electron to the 2p orbital) came from?

Anyone knows? Now I'm unsatisfied.