A green organic-inorganic laboratory experiment has been developed in which students prepare a self-assembling iron cage in D[subscript 2]O at room temperature. The tetrahedral cage captures a small, neutral molecule such as cyclohexane or tetrahydrofuran. [Superscript 1]H NMR analysis distinguishes captured and free guests through diagnostic chemical shifts, splitting patterns, diffusion coefficients (using DOSY), and the appearance of captured hydrophobic molecules in D[subscript 2]O even when free guests are insoluble in water. Students are invited to test their hypotheses about guest binding and to perform control and competition experiments. All of the reagents are commercial, the 23-component self-assembly is complete in 1 week at room temperature (requiring neither workup nor purification), and the product solution is intensely purple. In their laboratory reports, students effectively related organic chemistry with molecular self-assembly, supramolecular chemistry, and host-guest interactions.