While many fullerenes obeying the isolated pentagon rule (IPR) are experimentally known, isomers which violate this rule may become accessible via endohedral encapsulation of a guest molecule. Density functional theory computations predict a lower energy of non-IPR endohedral noble gas fullerenes over IPR analogues, specifically when C60 encapsulates a Xe dimer! So, the guest defines the shape of the carbon fullerene.