Two figures are congruent when a sequence of rigid motions carries one exactly onto the other. Because rigid motions keep every distance and angle, congruent triangles have three pairs of equal sides and three pairs of equal angles, and the converse holds too. You do not need all six: SSS, SAS and ASA each force a sequence of rigid motions that lands one triangle on the other. SSA and AAA do not, and the crew's cardboard strips show why.