If one believes that some of the "real" crop circles are not made by planking (human creation) then there are a number of interesting theories to explain how the crops are bent. Rapeseed, one of the favorite mediums, is a very brittle plant which usually will break if the long narrow stalks are bent. While there are many broken stalks in the crop circles, many plants exhibit a very unusual phenomenon where the "nodes" are bent, not broken.
Just like bamboo grows in segments, rapeseed stalks have segments which are connected by a special formation called a node which has the ability to be somewhat dynamic and orient the growing plant to maximize exposure to the sun. The node is not flexible, but it does have active cells that can grow quicker on one side or the other to change the plants orientation.
In "real" crop circles it has been observed that the nodes are often bent -- sometimes at 90 degrees -- and this accounts for the horizontal positioning that makes the designs of the crop circle. Other nodes exhibit holes in the nodes, while surrounding crops show none of these changes.
One good theory is that some sudden and extreme heat source has cooked the moisture inside the crops, causing the nodes to become soft (as if steamed from the inside) and, in some cases, to pop open under pressure. Indeed this has become the objective method for determining if a crop circle is "real" and samples of the plants are now routinely gathered for this assessment.
