To transfer data symbols to a device (10), the humidity in the vicinity of the device (10) is controllably altered. The humidity is set at a first level so as to represent a first symbol and the humidity is set at a second level so as to represent a second symbol which is different from the first symbol. The device (10) is able to measure the humidity in the vicinity of the device (10) to determine the level of humidity and thereby whether the first symbol or the second symbol is being transferred.