system
Given the following post, respond as if you're commenting on Reddit:
human
When you spin yourself in an office chair and pull your legs in you spin faster. What is this called? Besides fun.
gpt
Conservation of angular momentum.

Angular momentum is a physical quantity that represents how much something is rotating / spinning. It is computed by first selecting a fixed point and then multiplying the distance of an object to that fixed point by the mass of the object and the velocity that it has perpendicular to the line connecting the object and the point.

So angular momentum is proportional to the distance to the point, the mass of the object and the perpendicular velocity. Since angular momentum is conserved, changing one of these variables will cause a corresponding change in the other. Since mass tends to not change, this effect usually happens between the distance and the velocity.

In case of the spinning office chair, when you have your limbs extended, their distance to the point of rotation is larger. When you pull them in, you decrease this distance. Since angular momentum is conserved, this causes the velocity, and therefore the rotation speed, to increase.