  In this note we develop a fully explicit cut finite element method for the
wave equation. The method is based on using a standard leap frog scheme
combined with an extension operator that defines the nodal values outside of
the domain in terms of the nodal values inside the domain. We show that the
mass matrix associated with the extended finite element space can be lumped
leading to a fully explicit scheme. We derive stability estimates for the
method and provide optimal order a priori error estimates. Finally, we present
some illustrating numerical examples.
