How to auto focus on the nikon d3100 when taking a video?

If i'm using my nikon d3100 camera to take a video how do i auto focus? I know how to do it when taking pictures just not sure how to when using video

You can't.

Maybe it is just me, but it really amazes me when people buy a product and then expect it to do things outside its specifications.

Yes, some Nikon dSLR cameras can auto-focus when shooting video, but not a quickly as when shooting stills. Why. The ultra fast auto-focus sensors are in the viewfinder and with the mirror locked up necessary when shooting video a slower auto-focus scheme is used.

Here is something you may want to read if you intend to shoot more video.

http://www.luminous-landscape.com/reviews/camcorders/cam-cam.shtml

This is what most people use to focus their video when shooting video with a dSLR

http://www.luminous-landscape.com/reviews/accessories/hoodloupe-3.shtml

Using such a magnifier makes shooting video on a dSLR much more labor intensive.