# Connected and strongly connected Feynman diagrams

1. Recently I read, that only connected Feynman diagrams give contribution of nonzero values into the scattering amplitude. Why it is so and what is the physical sense of connected diagrams (due to their definition in Wikipedia)?

2. Also, I don't understand why strongly connected (=one-particle irreducible) Feynman diagrams are so important in scattering theory. By the other words, I don't understand why do we cut off one of the internal lines in the Diagram and does it relate to some physical process.