  The aim of this contribution is to illustrate two basic aspects of quantum
mechanics applied to the neutral kaon system. We first describe a recent
quantitative formulation of Bohr's complementarity principle for free--space
evolution of single kaons and entangled kaon pairs. We then show that the
neutral kaon system is also suitable for an optimal demonstration of the
``quantum eraser'', including its operation in the ``delayed choice'' mode. In
our discussions, strangeness oscillations play the role of the traditional
interference pattern linked to wave--like behaviour. The role of the two
interferometric paths taken by particle--like objects is played by the
differently propagating $K_S$ and $K_L$ components. Their distinct decay widths
provide a quantum ``mark'' which can be erased by appropriate strangeness
measurements.
