By Sally Tamarkin Popcorn might be the world's most versatile snack. It can be dressed up with cheese, caramel, butter, salt or more exotic flavors. It can be sweet, savory or both, and it's easy to grab by the handful for quick transport to the mouth. But popcorn is the Dr. Jekyll and Mr. Hyde of snacks — one minute it's the healthiest choice around (Low in calories! Whole grains!), the next it's getting banned for containing trans fats. What the heck is going on? Here's the deal: Kernels that are simply air-popped (in a brown paper bag in
