The Japanese industry ministry is reportedly investigating an incident in which an iPod Nano "overheated and discharged sparks." Reuters quotes the ministry "The battery part of the product overheated while being charged and sparked," Apple's iPod safety manual states "When you're using iPod or charging the battery, it is normal for iPod to get warm. The exterior of iPod functions a a cooling surface that transfers heat from inside the unit to the cooler air outside." As such, iPods have been known to overheat when placed in cases that do no allow adequate ventilation.