Spinny thing will stay spinny unless you apply torque to it. Just like moving thing will keep moving unless you apply force to it.
The farther out the mass of the thing is from the rotation axis, the harder it is to change how fast it spins. This “how hard is it to change its spinning” is the moment of inertia. Think of it like what the mass is to linear movement.
Just like normal momentum is velocity * mass, angular momentum is now angular velocity * moment of inertia.
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