A modification of the mixed-aldol synthesis of dibenzylideneacetone, prepared from acetone and benzaldehyde, is described wherein acetone is replaced with a series of cyclic ketones with ring sizes of 5-7 carbons. The structural variations in the resulting conjugated ketones produce regular variations in the UV-vis absorption spectra. The choice of the best sunscreen agent based on these variations is straightforward, and by considering simple molecular orbital diagrams in the context of the molecular geometries, students can go the next step and understand why the variation occurs. (Contains 2 tables and 3 figures.)