A facile, discovery-based experiment is described that introduces aliphatic substitution in an introductory undergraduate organic chemistry curriculum. Unlike other discovery-based experiments that examine substitution using two competing nucleophiles with a single electrophile, this experiment compares two isomeric, competing electrophiles (primary and secondary halides) with a single nucleophile (phenoxide ion). The question posed to students at the start of the experiment is what mechanism explains formation of the observed ether product from reaction of the phenoxide with an alkyl halide. If the reaction proceeds via an S[subscript N]1 mechanism, then the major product will come from reaction of the secondary halide. If the reaction proceeds via an S[subscript N]2 mechanism, the major product will come from reaction of the primary halide. In this experiment, students are able to discover that the primary halide reacts faster than the secondary halide by a roughly 4:1 margin. This discovery allows students to conclude that the reaction proceeds via an S[subscript N]2 mechanism.