TSA liquids rule: What to know about 3-1-1 before you head to the airport

It's the rule every passenger has to consider when packing for a flight.For 20 years now, the Transportation Security Administration has enforced rules on the liquids passengers can bring in carry-on bags.This policy is the reason you can't bring a full-size bottle of shampoo or sunscreen through a TSA checkpoint. It's one of the biggest reasons bags get pulled for secondary screening.Last year, the TSA stopped requiring flyers to take off their shoes, but the liquids rule remains one of the biggest hurdles travelers still face when passing through security.Travelers wait in a TSA line at Chicago's O'Hare International Airport (ORD). KAMIL KRZACZYNSKI/AFP/GETTY IMAGESBut what exactly are the TSA liquid rules, and why do they exist? And, most importantly, what does this mean for packing ahead of your next trip?Here's a quick rundown.TSA liquids rules: Why do they exist?The TSA's restrictions on liquids date back to 2006. In the wake of a thwarted terrorism plot overseas, the agency started restricting the liquids, gels and aerosols passengers could bring in their carry-on baggage.Scores of other countries around the world put similar policies in place in the name of aviation securit...