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The taming of cyclobutadiene was accomplished in the interior of the hemicarcerand 1. Cyclobutadiene is a stable compound with a singlet ground state when it is synthesized in the interior of 1. In order to synthesize the incarcerated cyclobutadiene and to characterize its structure, one bimolecular, three photochemical, and two thermal reactions were carried out in the interior of 1. The authors consider it both realistic and useful to regard the internal phase of carcerands and hemicarcerands as a new state of matter.