You can set notifications to find out when your collaborators have modified your spreadsheets, and learn what sheets or cells they've modified. You can also choose how often you'd like to be notified.
Great website where you enter your iPhone or Android phone's IMEI and find out when your repair and service warranty is due and whether your device is locked on unlocked by the carrier.
I will show you how you find out if a website of your choice is using cookies. If the result is positive, you also find out which cookies are being used and what's their purpose.