In or out? Contrarily to the well-established tendency of 1-methylcyclopropene compounds to isomerize to the corresponding methylenecyclopropanes, the spiro-derivative MeCP is refractory towards its conversion into MCP. The reasons of this unexpected behavior were unraveled by high-level computations, which showed that in the case under exam the endocyclic C−C double bond isomer is more stable than