I'm thinking of a number...

Affordable brainwave sensors could make typed passwords obsolete (The Verge)

The last hurdle involved determining what specific mental tasks would be best-suited to this type of authentication — the team wanted the interaction to be as user-friendly as possible. To find the most suitable tasks, the team the brainwaves of test subjects performing seven different mental activities to authenticate their identify. Researched showed that the best tasks for this setup were ones that users didn't mind repeating on a daily basis — the tasks need to be easy, but not too boring.

Interesting sort of behavioral biometric of the brain.