  We consider new objects in bosonic open string theory -- ND tadpoles, which
have N(euman) boundary conditions at one end of the world-sheet and D(irichlet)
at the other, must exist due to s-t duality in a string theory with both NN
strings and D-branes. We demonstrate how to interpolate between N and D
boundary conditions. In the case of mixed boundary conditions the action for a
quantum particle is induced on the boundary. Quantum-mechanical particle-wave
duality, a dual description of a quantum particle in either the coordinate or
the momentum representation, is induced by world-sheet T-duality. The famous
relation between compactification radii is equivalent to the quantization of
the phase space area of a Planck cell. We also introduce a boundary operator -
a ``Zipper'' which changes the boundary condition from N into D and vice versa.
