how did scientist come to know that the outer core is liquid

Scientists came to know that the Earth's outer core is liquid primarily through the study of seismic waves generated by earthquakes. They observed that S-waves (secondary or shear waves), which can only travel through solid materials, do not pass through the outer core. Instead, an S-wave shadow zone is created, indicating that the outer core is not solid but a liquid, as liquids do not transmit shear waves. Furthermore, P-waves (primary waves), which can travel through both solids and liquids, do pass through the outer core but change speed and direction, reinforcing the inference. The disappearance and reappearance of S-waves at different layers inside the Earth helped confirm this. Additional support comes from laboratory experiments simulating the high-pressure conditions of the outer core, which indicate that iron would be in a liquid state at those conditions. The presence of Earth's magnetic field is also explained by the movement of molten iron in the liquid outer core, according to the dynamo theory. In summary, the main evidence for the liquid outer core is the inability of S-waves to travel through it, combined with P-wave behavior, lab experiments on iron under pressure, and the Earth's magnetic field generation mechanism.