  As the temperature of a cooling object decreases as it relaxes to thermal
equilibrium, it is intuitively assumed that a hot object should take longer to
cool than a warm one. Yet, some 2,300 years ago, Aristotle observed that "to
cool hot water quickly, begin by putting it in the sun". In the 1960s, this
counterintuitive phenomenon was rediscovered as the statement that "hot water
can freeze faster than cold water" and has become known as the Mpemba effect;
it has since been the subject of much experimental investigation and some
controversy. Although many specific mechanisms have been proposed, no general
consensus exists as to the underlying cause. Here we demonstrate the Mpemba
effect in a controlled setting - the thermal quench of a colloidal system
immersed in water, which serves as a heat bath. Our results are reproducible
and agree quantitatively with calculations based on a recently proposed
theoretical framework. By carefully choosing parameters, we observe cooling
that is exponentially faster than that observed using typical parameters, in
accord with the recently predicted strong Mpemba effect. Our experiments
outline the generic conditions needed to accelerate heat removal and relaxation
to thermal equilibrium and support the idea that the Mpemba effect is not
simply a scientific curiosity concerning how water freezes into ice - one of
the many anomalous features of water - but rather the prototype for a wide
range of anomalous relaxation phenomena of broad technological importance.
