When lit from the front, this 1,600 year old Roman goblet looks green. Lit from behind, it looks red. It probably would change colors depending on what liquid it held, although nobody's willing to try that.
The secret turns out to be nanoparticles of silver and gold just 50 nanometers in diameter, or 1,000 times smaller than a grain of salt. The particles are so sensitive to composition that this sort of tech "may one day make its way into handheld devices for detecting pathogens in samples of saliva or urine, or for thwarting terrorists trying to carry dangerous liquids onto airplanes." Nice job, ancient Romans, nice job.
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