Sterile swabs collect material from airplane seats, tray tables, and armrests. The collected swabs — labeled carefully and shipped on ice — go to a laboratory that measures peanut protein content in each sample. When results came back, one seat showed 2.5 parts per million of peanut residue while the other three had none. That single detectable trace converts an invisible surface contaminant into a measurable quantity, which is exactly how testing a surface reveals whether a substance that causes bad reactions is present.