Help with quantic physics please!?

Question

Can anyone explain me the ending of this video? Why does that happen?

http://www.youtube.com/watch?v=vfkdzNN2VLo

Answer 1

In order to see which slit the electron went through, we have to interfere with it. Light or something must bounce off the electron to our  detector to tell us which slit it went through. Once you do this, you have fixed (by your observation) which slit it goes through and you get the 2 line picture on the screen. If you don't look at it, you get the wave picture. Weird, huh? How does the electron know you are looking at it??