The average air traveller carries several types of liquid, the overall amount of which is multiplied when she/he travels with companions.
It is obviously impossible for each liquid to be analysed as we pass through security.
Combinations of certain liquids produce explosives; and a large, mostly-empty bottle provides an ideal mixing-vessel. Or bomb.
It would be helpful if all airport authorities made clear that little drops must be decanted into little bottles.
Trish Miller, by email