Decoding the riddle of Y(4260) and Zc(3900).

The observation of Zc(3900) by the BESIII Collaboration in the invariant mass spectrum of J/ψπ± in e+e-→J/ψπ+π- at the center of mass 4.260 GeV suggests the existence of a charged DD*+DD* molecular state with I(JP)=1(1+), which could be an isovector brother of the famous X(3872) and an analogue of Zb(10610) claimed by the Belle Collaboration. We demonstrate that this observation provides strong evidence that the mysterious Y(4260) is a DD1(2420)+DD1(2420) molecular state. Especially, we show that the decay of this molecule naturally populates low momentum DD* pairs and leads unavoidably to a cusp at the DD* threshold. We discuss the signatures that distinguish such a DD* cusp from the presence of a true resonance.