How can I completely clear my smartphone of all information?

If I loan my smartphone to a stranger who needs to make a call, is my information safe?

• I was at an airport when a stranger asked to borrow my phone to call the people who were supposed to pick her up. I loaned her my phone and she spoke, in a language foreign to me, to these people. She then left. I later got to wondering: Is there any risk to me in my good Samaritan tendencies in letting someone use my smartphone? Can someone access my Google Drive documents, for instance, while pretending to use my phone? (BTW, it was due to her waiting at passenger pickup with no baggage, claiming to come in from international flight in domestic terminal, etc. that started my train of thought. She probably was embellishing her story for reasons other than nefarious, and a nation-wide burglar operation didn't strip my house while I was gone.)

• Answer:

  It would be very difficult to steal only information without you seeing "something" going on, assuming you were in plain view of the phone the whole time. It's quite difficult to manipulate the phone UI while holding it up to your ear. It would be far more likely that a thief would simply shove you aside and run away with the phone; this puts all the information on the phone at risk, especially if it were unlocked when you handed it over! If I were "loaning" an iPhone to a stranger, I would offer to dial the number for them, and would do so via Siri leaving the phone locked, before handing it to anyone. This would prevent immediate access to the phone data in most cases and reduce the chances of theft. (My iPhone is configured with the "Find my Phone" feature, and with a data wipe after too many failed PIN attempts.)