How did scientists come to know thet the outer core is liguid?

Scientists determined that the outer core is liquid primarily through the study of seismic waves generated by earthquakes. When these waves travel through the Earth, they behave differently in solid and liquid materials. Seismic S-waves, which can only move through solids, do not pass through the outer core, indicating that it must be liquid. Additionally, the behavior of P-waves, which can travel through both solids and liquids, slows down when they enter the outer core, further supporting the conclusion of a liquid state.