  From a study of the lineshape of the $X(3872)$, the LHCb collaboration
measures a sizeable negative effective range. This cannot be reconciled with a
shallow $D\bar{D}^*$ bound state hypothesis. Based on Weinberg's compositeness
criterion, together with a theorem by Smorodinsky, it follows that the $X$ has
to have a compact hidden charm structure, most likely a tetraquark, interacting
with unbound $D\bar{D}^*$ pairs via short-distance color forces. This
conclusion is strengthened by the general pattern recently emerging for exotic
mesons.
