Imagine a tiny drop. It contains water, the fizzy antiseptic hydrogen peroxide, and a common, yellowish chemical called glyoxal. When the droplet is exposed to light, a cascade of reactions occurs, producing new materials. These reactions occur on the surface, where liquid meets air. Scientists didn’t have many details about the reactions until Dr. Xiao-Ying Yu from DOE’s Pacific Northwest National Laboratory and her colleagues took up the challenge. They got the details using an imaging mass spectrometer that typically boil liquids away in vacuum. They found that the reactions don’t stop when the light fades. They also saw how the resulting products mixed on the surface and reacted with more than 40 water clusters.