Thermodynamic PV-diagram: Why does a critical point =inflection point?

http://en.wikipedia.org/wiki/Critica...rmodynamics%29 Look here please.

Why is the first and 2nd order derivatives of pressure with regards to volume equal zero at the critical point for an isotherm phase curve? Why/how is it possible to expand a fluid at its critical point without any change in pressure?

thanks for all help :)