Set Theory

Think of this crossword as a Venn diagram consisting of two sets--one right side up, the other rotated 180 degrees. The shaded section in the middle represents the overlap of the two sets, with answers in this area working for both sets. How is this possible? All will become clear as you solve the puzzle. The puzzle also contains 12 clues related to articles that appeared in Scientific American in 2006--one for each month. Visuals from the referenced articles frame the grid. Find the answers at www.sciam.com/ontheweb and in next month's issue.