  Chandra and XMM-Newton observations of many clusters reveal sharp
discontinuities in the surface brightness, which, unlike shocks, have lower gas
temperature on the X-ray brighter side of the discontinuity. For that reason
these features are called ``cold fronts''. It is believed that some cold fronts
are formed when a subcluster merges with another cluster and the ram pressure
of gas flowing outside the subcluster gives the contact discontinuity the
characteristic curved shape. While some edges may not arise directly from
mergers (e.g., A496, Dupke & White, 2003), this paper focuses on those which
arise as contact discontinuities between a merging subcluster and the ambient
cluster gas. We argue that the flow of gas past the merging subcluster induces
slow motions inside the cloud. These motions transport gas from the central
parts of the subcluster towards the interface. Since in a typical cluster or
group (even an isothermal one) the entropy of the gas in the central regions is
significantly lower than in the outer regions, the transport of the low entropy
gas towards the interface and the associated adiabatic expansion makes the gas
temperature immediately inside the interface lower than in any other place in
the system, thus enhancing the temperature jump across the interface and making
the ``tip'' of the contact discontinuity cool. We illustrate this picture with
the XMM-Newton gas temperature map of the A3667 cluster.
