A scientific breakdown of Signs from

• In 1991, British artists Doug Bower and Dave Chorley confessed to having created the [crop circles] that started the craze, and several Web sites now provide detailed instructions on how to make your own.

• Let's consider why man-eating is an unlikely motivation. First, our proteins and fats and nucleic acids almost certainly would not agree with an independently evolved alien digestive system. Second, any civilization that can travel between the stars necessarily has access to tremendous reserves of energy and materials. Finding a snack is not going to be a problem. And even if terrestrial flesh were a unique taste treat, wouldn't cows make a better choice? They have a lot more meat and put up less of a fight.

• A civilization that can travel through light years of empty space probably would not need to send scouts onto the ground to find conveniently located farms and then knock down their corn.
http://www.discover.com/science_news/features/gthere.html?article=feat_signs.html