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The
earth is not a solid ball. The outside is a thin shell or crust of rock.
Under the crust is a thick mantle of rock, so warm that it flows very
slowly, like molasses. The heat is left over from the planet's formation
and is also generated by nuclear reactions deep inside. The Earth's
core is 3,000 km (1,800 miles) down. The outer core is so hot that it is
always molten. Despite its great heat, the inner core is solid - pressure
there is so high, it cannot melt.
 Because of its dense core of iron, the Earth is like a giant magnet. Its
magnetism is so powerful that it affects every other magnet on its
surface. When a compass needle (a tiny magnet) is allowed to swing freely,
it always stops with one end pointing to the magnetic north pole and the
other to the magnetic south pole.
The
best aid to find your way on land or sea is a compass. Compass needles are
tiny magnets that are made to point north by the Earth's magnetic field.
However, the needle points to the magnetic north pole, not the true
geographic North Pole, which is about 1,000 km (600 miles) away. When
using a compass, you need to know the declination - the difference in
direction between true and magnetic north - and then make adjustments.
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