An undergraduate laboratory experiment has been developed for the synthesis of a reactive cross-linker that is useful in various highly visual demonstrations of organic reactivity, click chemistry, and biobased polymer synthesis. A triazolinedione-based cross-linker, which is known as a useful reagent for click chemistry, can be obtained from biobased isophorone diisocyanate (IPDI) in three synthetic steps: two steps to elaborate a urazole precursor, and finally a mild oxidation of this urazole to the highly reactive triazolinedione click reagent. As triazolinediones are strong chromophores, the progress of the oxidation step can be visually monitored by the appearance of a bright red-pink color. Students can perform either all three synthetic operations or just the final oxidation step, starting from a prepared batch of the precursor. Alternatively, for less experienced students, the reactive cross-linker or click reagent can be prepared beforehand by a lab assistant and used as such. Students used the freshly prepared click reagent to covalently cross-link various unsaturated plant oils into a polymer network. The gelation of the plant oil into a "plant foil" can be visually monitored, as the bright red-pink color of the cross-linker disappears over time. The purity of the synthesized cross-linking reagent can also be assessed by students by using a simple titration experiment. Students can rely on various practical first and second year undergraduate lab skills, but at the same time, they arrive at a tangible and visual experience of chemical reactivity.