[Note: This was originally titled "Windows Hello face recognition is vulnerable to the Jedi mind trick" but many people overreacted to the the word "vulnerable".]
Here's a handy tip if you use Windows Hello face recognition, which is available on the Surface Pro 4, Surface Book and a number of other devices: If the device doesn't recognize you, use the Jedi hand wave.
Here's the deal: Suppose you walk up to your device and try to sign in with Windows Hello face recognition, but the recognition fails for whatever reason, say, because the lighting conditions aren't right. Even if you fix the problem, the face recognition has already determined that "This face in front of me is not anyone I know." In order to get it to re-evaluate the face, you have to make the device "see" a new face. One way of doing that is to move your head out of the field of view of the camera, and then bring it back in.
But another way to do this is to wave your hand slowly in front of the camera, as if you were performing a Jedi mind trick. (Fool your friends!)
Waving your hand in front of the camera interrupts the view of your face, causing the device to attempt to recognize this "new" face that suddenly appeared in front of it.
Once you get logged in, you may want to go to the Sign-in options settings page and select Improve recognition, so that it learns your new face.
But anyway, the point is: Jedi mind trick. It works in real life.