How do mentalists guess the picture you draw and conceal?

Mentalists have a trick where they guess what picture you have drawn and concealed in an envelope. An alternate method puts a screen between the mentalist and the subject. I've seen it done and the trick seems amazing. But I know it is a trick. How is it done?|||Well, I've got a few.





a) It's all rigged, and the 'volunteers' are set up and know what picture to draw.


b) They can give you subtle cues that you don't even realize you're receiving. For example I saw a show where a "mentalist" asked an audience to draw a picture. It was later revealed that he made the yin and yang symbol with his hand movements, and 60% of the audience had drawn it.


c) For behind-the-screen tricks, they could have people placed in the audience that give them cues as to what the person has drawn.





I'm not a mentalist so I can't really tell you for sure, but that's all I know :)