Have you ever heard of a device that has an "Airplane Mode?"
"Airplane mode" is a mode a lot of modern electronic devices have that turns off Wi-Fi, sound, and that sort of thing to prevent the device from sending signals that interfere with the airplane's navigational equipment.
What items are electronic and could plausibly have an airplane mode?
These are small, handheld items that you'd normally take on an airplane.
Phone signals can potentially interfere with airplane radios.
An iPad or similar tablet could potentially emit a Wi-Fi signal that could interfere with plane navigation.
A lot of handheld gaming devices, such as the Nintendo DS, also have something akin to Wi-Fi for communicating with nearby devices.