Earth keeps spinning because, absent a net external torque, what is conserved?

1. The explanation rests on a conservation law. 2. A nonzero net torque changes the angular momentum of an object. 3. With no net external torque, that change is zero. 4. Conservation of angular momentum follows. 5. We can now understand why Earth keeps on spinning. _Source: OpenStax College Physics for AP Courses 2e (CC BY 4.0), Ch 10 'Rotational Motion and Angular Momentum', sections 10.1-10.7_