I like the suggestion of letting the user decide on the activity. I found it a little unnerving the first time I came across it just after Halloween... it was a

I like the suggestion of letting the user decide on the activity. I found it a little unnerving the first time I came across it just after Halloween... it was as though some strange individual knocked on my door after weeks of stalking my friends, holding up pictures of them and asking, "Is this your friend Jane Doe? How about this one?" followed by 40 more pictures. Even just by saying, "We're trying out facial recognition, would you like a demo?" I would've felt more impressed rather than creeped out.