From a Time magazine analysis of the ease of use of liquid explosives in airplanes:
. . . The particularly devious innovation of the London plotters was their alleged use of liquid explosives, which are easily concealed in many of the items found in most travelers' hand luggage — perfume, hair gel, deodorant, medicines, drinks, toothpaste, lotions, and so on — and is extremely difficult to detect. Metal detectors will obviously miss them. While there have been some 'puffer' explosive-detection machines placed in some U.S. airports, they are few and far between — and aren't made to detect liquid explosives in sealed containers.
Liquid explosives also attack airline security's weakest point — the Transportation Security Administration screeners. They are the burger-flippers of the entire security system, and the chances of even the best of them visually identifying a liquid explosive in an innocuous bottle are slim — that's why Israel's Ben Gurion airport has a laboratory in the basement to conduct instant tests of liquids found on suspect passengers. If the U.S. system lacks sufficient technology to detect liquid explosives, and if it relies on the TSA screeners to ID possible terrorists, it is, at best, a wire mesh fence.