  Motivated by recent discussions of IR/UV mixing in noncommutative field
theories, we perform a detailed analysis of the non-planar amplitudes of the
bosonic open string in the presence of an external B-field at the one-loop
level. We carefully isolate, at the string theory level, the contribution which
is responsible for the IR/UV behavior in the field theory limit. We show that
it is a pure open string effect by deriving it from the factorization of the
one-loop amplitude into the disk amplitudes of intermediate open string
insertions. We suggest that it is natural to understand IR/UV mixing as the
creation of intermediate ``stretched strings''.
