We develop a Morse-like theory in order to prove the Weak Factorization Theorem, which states that any birational map can be factored into a sequence of blow-ups and blow-downs. In the above theory Morse function is replaced by $C^*$-action. Critical points corespond to fixed point components of this action and spherical modifications could be perceived as some elementary birational transformations as for example blow-ups or blow-downs.