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What Gwalk says should be taken seriously. It's a good idea to put all your portable devices in airplane mode which generally severs their connection to the outside world. HOWEVER: you may have devices that appear to have no way to turn off their wi-fi connection. For example, the designers of a 90-inch smart TV probably don't expect it to be operated in a passenger airplane, so it probably won't have an airplane mode that can be turned on. Nor are the designers of these gadgets sensitive to data caps, so they communicate willy-nilly with the outside world. You CAN sever their connections, however, by changing its Wi-Fi password to something incorrect.