  We discuss the "wrong"-sign DBI theory as a prototype for classicalization.
The theory lacks a UV completion and has to be defined with a fundamental UV
cutoff. We study a quantum-mechanical toy model with similar properties. The
model has a fundamental length scale and all physical states have momenta below
the inverse of this scale. We show that, despite the terminology, the
phenomenon of classicalization is of a quantum nature. Within the toy model it
consists essentially of tunnelling through a region that is classically
forbidden. The size of this region is proportional to the square root of the
energy and can be much larger than the fundamental length scale. We discuss the
implications for classicalization in scalar field theories.
