Depending on the retailer, the card you're using, and the value of your purchase, you might need to enter your PIN or provide a signature to complete the transaction.
And don't forget the best part of Apple Pay on the Apple Watch: You don't need the iPhone present to use it. You could leave your phone at home, go for a run using the Workout app on your watch, and then stop at Whole Foods for a smoothie on your way home.
Your Apple Watch locks with a passcode as soon as you take it off, so if you were to ever lose it, or it's somehow stolen, whoever gets it can't use Apple Pay without knowing your watch's passcode. (And you can disable the cards using the Apple Watch app on your iPhone.) There's a setting on the watch itself to all you to unlock the watch by unlocking your iPhone, so be sure to lock your iPhone with a passcode if you're going to leave that on--just in case you lose both at once!
I've gotten just as much attention using Apple Pay on my watch as I did using my iPhone when Apple Pay was brand-new, but I'm sure that will die down once the Apple Watch isn't quite so novel. Using it hasn't been awkward or weird, just cashiers asking, "Is that the Apple Watch?" By the time I smile and say yes, the transaction is over, and I'm on my way.
For an up-to-date list of which banks and retailers support Apple Pay, check out our Apple Pay guide. I've used it at Walgreen's, Subway, McDonalds, Panera, and even the soda machine in our office--buying something with a flick of my wrist feels futuristic every time, and I'm hooked.
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