What force causes the earth to spin on its own axis

The gravitational force from the sun causes the earth to orbit around it,
however what force causes the earth to spin on its own axis?

Earth has got its angular momentum from it's birth. Earth was created from dust. This dust came together to form bigger and bigger lumps of rock. These lumps had attracted with it's gravity other material and it grew bigger. And there it got it's initial spin. When gravity attracts another object it basically this object free falls toward larger object. Because everything is in motion that object which is being attracted starts to circle around larger object. This principle causes material to form a disk around centre of mass. In time this disk will form a one object which is still rotating. So it's been gravity which started Earth spin. Now it is just spinning along and slowing down slowly.

Actually there is no perfect explanation for that, but it's explained that every celestial body in nature revolves around the bigger body responsible for it's orbit, and earth with an atmosphere has trade winds and tidal conditions due to the presence of heavy ocean beds, this tends to rotate the earth, which began at it's birth, but it hasnt stopped rotating yet cause there is no frictional resistance against its movement to oppose it, but lately it's found that the earths rotation has actually reduced in rate from the past!

Angular Momentum. It's not a force (there are only 4 types of forces), so it could be argued that the gravitational attraction that caused the collisions of the early earth rocks is responsible for getting the earth to start spinning, and there's simply nothing that's really slowing it down (not drastically slowing it down, like Mercury or the Moon)

http://www.universetoday.com/14491/why-d…

Technically it's conservation of angular momentum, but more fundamental than that is gravity of the earth. If not for earth's gravity, every part of the earth would fly away along straight paths instead of circling the center.