  In this paper, two examples of sonic interfaces are presented. The first
example shows the case of sonic interfaces as weak discontinuities in
self-similar shock configurations of unsteady Euler system. The second example
shows the case of sonic interfaces as regular interfaces in accelerating
transonic flows governed by the steady Euler-Poisson system with self-generated
electric forces. And, we discuss analytic differences of the two examples, and
introduce an open problem on decelerating transonic solution to the steady
Euler-Poisson system.
