Pennsylvania is one of the strictest states when it comes to sell-by dates on milk: It has to be within 17 days of the date the milk was pasteurized. Milk can’t be sold after that, so it’s either thrown or given away. The policy isn’t about food safety. The Pennsylvania Department of Agriculture says it’s a quality issue. But a physics professor at Indiana University of Pennsylvania thinks he’s accidentally hit on a better way to tell how fresh your milk is. Greg Kenning and his students were working on a completely different project that involved embedding magnetic nanoparticles of cobalt into the semi-metal antimony. And they noticed some interesting things about the magnetic and electronic properties of these elements as they decay with time and temperature.