What is a liquid?

Airport security get’s stranger by the minute. Passing through London Heathrow airport on a connecting flight from Germany to the US I pass a glass vitrine with various items, including things that look like toothpaste, creams, gels and fluids. Above the glass box is a sign saying “What is a liquid?”. In the past I thought this is more a physical definition of one of the states material can be in. But not only seems this glass box try to re-define what a liquid is, it also seems to include things that are probably not in a liquid form and therefore defying what generally has been accepted internationally as a definition for liquids. What can I say, airport security not only often doesn’t seem to make sense now it is also changing the laws of physics.