The layer of the Earth that causes it to act like a giant magnet is the outer core.

Here's why:

* Composition: The outer core is made up of liquid iron and nickel.

* Convection Currents: The intense heat from the Earth's inner core causes the liquid metal in the outer core to move in swirling convection currents.

* Electric Currents: This movement of electrically conductive material generates electric currents.

* Magnetic Field: These electric currents, in turn, create a magnetic field that extends far out into space, forming the Earth's magnetosphere.

So, the outer core's swirling, electrically conductive liquid metal is the key to Earth's magnetic field.