This is a story about how urea-based compounds can sense ketones such as acetone and cyclohexanone, which are used in the purification of certain explosives and whose high vapor pressure make them useful for detection. Over the past few days I made this stop motion animation to play as the narrator (aka the graduate student working on this research) describes how ketones interact with urea. She explains that the lone pairs of electrons on the ketone's carbonyl oxygen "grab" urea's hydrogens. My collaborators suggested the arms literally reaching out and grabbing the H's, which then set the tone for the rest of the animation. The ensuing hydrogen bond between oxygen and hydrogen leaves the N-H bond to exist in a "lone pair-like" state. As my former Chem 151 students hopefully remember, a lone pair is believed to take up more space than a bond, therefore creating more repulsion of the neighboring bonds. The bond angles adjust to accommodate this change, and voila. This subtle change in structure is amplified through the connected polymer in such a way that both the fluorescence and refractive index properties of the compound are changed, and these changes can be measured. I never considered lone pairs to be such bullies but I guess they are kind of lone rebels. I'll bet Mala Radhakrishnan could write a poem about this...