Why have scientists reasoned that earth's outer core is liquid?

in different parts of Earth's interrior, there are solid and liquid layers. Why is this? scientists believe that as you travel further through Earth's core, the temperature increases as well as pressure. The mantle, the closest layer to the crust, is solid due to its temperature. However, the outer core is made up of liquid because of its temperature as well, but the inner core isn't. It may be the hottest part of Earth, but it's under so much pressure that it remains a solid state.