Here's why:

* Composition: The outer core is primarily made of iron and nickel, with smaller amounts of other elements.

* Temperature and Pressure: The intense heat and pressure deep within the Earth cause the iron and nickel to melt.

* Convection: The liquid outer core is in constant motion due to convection currents. These currents are driven by heat from the inner core, and they generate Earth's magnetic field.

While the outer core is liquid, it's not like the water in a lake. It's extremely dense and under immense pressure.