Purchases are made using your saved payment information, held in your Google Play Store account.
If you'd like to avoid any accidental purchases on your Android device, simply open the Google Play Store app and follow the steps below.
1. Tap the 'Menu' button in the top left corner corner of the Google Play Store search bar
2. Select 'Settings'
3. Scroll down to the 'User Controls' section and select 'Require password for purchases'
4. Select the first option: 'For all purchases through Google Play on this device'
Automatic In-App Purchases are now disabled and a password will be required to make any purchases.
Please note: the instructions above may vary according to the device and software version you are using. You can double check your device user manual for further information.