Airlines security policies post 9/11
Limited quantity of liquids
After the foiled attack on a trans-Atlantic flight in 2006, airlines security has become tighter regarding carrying liquid on board. Al Qaeda had planned to blow up several flights in mid-air using liquid explosives. These are binary explosives -- they don't look like explosive compounds separately, but when mixed with two different liquids together it can be very powerful.
After a number of extensive explosives testing, TSA has declared that liquids, aerosols, and gels in small quantities are safe to bring on flight. TSA advises declaration of larger quantities of liquids before inspection at the checkpoint.