Turns out, you can! To enable WiFi without using a cell signal, switch to (or keep your phone in) airplane mode and just turn on the WiFi. You won't be able to send text messages or make phone calls, but every other function and application that works with WiFi will work on your phone (including Words With Friends! Goodbye, in-flight boredom). While this is obviously useful in flight, it's also useful if you're in an area with poor phone reception and don't want to drain the battery, a foreign country where you don't want to pay hefty out-of-network charges while visiting, or if you don't want to be distracted by phone calls or text messages but want to send email or use other functions on the phone.
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