How did scientist come to know that outer core is liquid?

Scientists determined that the Earth's outer core is liquid through the analysis of seismic waves generated by earthquakes. When these waves travel through the Earth, they behave differently depending on the state of the material they encounter. Primary waves (P-waves) can pass through both solid and liquid, while secondary waves (S-waves) cannot pass through liquids. The absence of S-waves in specific regions indicated that the outer core is liquid, as they did not travel through that part of the Earth.