To prevent yourself from unintentional and accidental purchases as well as purchases that can be made by somebody else who uses your phone without your knowledge (e.g.: children or someone else), you can set a PIN-code. You can use the option if you're running Google Play version 3.1 or higher on your device.
To set a PIN-code for purchasing, perform the following activities:
1. Open Google Play on your device and press Menu → Settings → Set or change PIN.
2. Enter a PIN-code and press OK.
3. Then you'll need to re-enter your PIN to confirm.
4. Check the box for the Use PIN for purchases option.
Whenever you try to purchase in an application, you'll be prompted to enter this PIN code before you can complete your purchase. You can find more details here: http://support.google.com/googleplay/bin/answer.py?hl=en&topic=1046718&answer=1626831
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