How to set a plane's normals facing both side?
 

Here is a picture that might explain my question. It's part of a flower vase. I just want to know how to get that effect? When I build a plane, It only has normals pointing from one side, not both. I know that may be a quite stupid question, but I just cannot figure it out myself :( Thank you for helping!

http://img.photobucket.com/albums/v3...ormal_prob.jpg

Not a stupid question. A face only has one normal, but with the default settings a texture will appear on both sides of a face. So unless you've changed the settings both sides will render.

Thank you for answering :)
I know in Maya it works fine, but actually I'm trying to make a Sims 2 (a game, in case you don't know) object. The object showed in the picture is extracted from the game. Here is picture that shows what the problem will be if a plane has only one normal (left is in game effect and right is in maya). You can see the stamens not display correctly, because the normals of petals is pointing into the middle of the flower. However, if I simply reverse the normals, the petals on far side won't show because they are pointing away from the camera. Thus in order to make it work, I need to have normals on both side of the plan, as the picture above :confused:
http://img.photobucket.com/albums/v3.../vase_prob.jpg

If there's no specific poly limit, you could try to make duplicates of the faces, place them on top of the original ones and reverse the normals... Just a thought, not sure if it'll work...

Yes, duplicating and reversing the duplicate's normals is what we do if the engine doesn't support double-sided faces.