If it weren't for the fact it's classed as dangerous goods I'd love to take a vial of Caesium on board..... " Now it's a liquid, now it's a solid ) "

Failing that I will nip into the lab tomorrow and try and find something with a Triple Point around room temperature, and then place it in a pressurized container at it's TP. That would mess with their heads. " Now it's a solid, now a liquid, now a gas"