  The existence of the $\Theta^+$ pentaquark requires a peculiar mechanism to
explain its stability. Looking at quark clusters, notably diquark and triquark
configurations, such a mechanism may be found in the color-magnetic interaction
between quarks. It is possible to understand why the $\Theta^+$ is unique.
Chiral dynamics, in particular the ease of pion emission, will render other
members of the same flavor antidecuplet, such as the $\Xi^{--}$ very unstable.
