Yes. Given three lengths, e.g. 6 cm, 7 cm, 8 cm, there is only one triangle that can be made - hence SSS being a proof of congruence. This makes it rigid - it can't be transformed nto another...
Liquids are not rigid because the intermolecular forces of attraction between its molecules are not strong enough to hold them as closely as solids, but they are strong enough to ensure that liquids...