[freehunter]

>Neither can you.

That's the point. It's not something I know, it's something I am and only I am that thing.

And unlike a password, if you want my fingerprint you have to be physically near me, and if you want to authenticate as me you need my authentication hardware. A Brazilian hacker isn't going to unlock my iPhone without first flying to the US and then locating me in both space and time to gain access to my fingerprint and my phone simultaneously. But with a password, they could easily go to www.gmail.com and type whatever they want from the comfort of their own home.