Walter writes, “I read a story about a teenager who ran up a $2,000 phone bill because they thought they were using Wi-Fi but they were actually on the cellular network. I didn’t catch the whole story, but I know it had something to do with iPhones. Do you know what’s going on?”

We heard about that story too, Walter, and all we can say is, “Yikes!”

It sounds like it wasn?t entirely the kid’s fault, though. There’s a new feature in iOS 9 called Wi-Fi Assist that was at least partially responsible.

Wi-Fi Assist helps to make sure you have a solid data connection when you’re using Wi-Fi, even if your Wi-Fi connection is really weak. For example, if you’re in a part of your house that is far away from your wireless router, Wi-Fi Assist will automatically switch your phone over to 4G cellular data to keep you connected to the internet.

This is great for people who have unlimited data plans, but it could run up your bill pretty quickly if you have a data cap and you’re not paying attention to how much you’re using.

For tips on keeping your data usage low, check out our other articles here, here and here.

If you’re worried about Wi-Fi Assist eating up your data, here’s how to turn it off:

• Open the Settings app
• Tap Cellular
• Scroll all the to the bottom of that menu
• Tap the toggle button next to Wi-Fi Assist

In the Cellular menu, you’ll also see a list of all of the apps on your phone that use cellular data. Next to each one is a toggle button to turn data for it on or off. It’s a good idea to look through this list to see if there are any using data that don’t need to be, especially ones you rarely use anymore.