system
Given the following post, respond as if you're commenting on Reddit:
human
Reading the post about Uranus and it’s 90 degree axis - Why do planets rotate to start with?
gpt
Planets rotate because of the Conservation of Angular Momentum. The gas and dust that forms planetary systems has some net rotation to it before it starts contracting. There are a lot of reasons for this, but basically it's very hard to not have angular momentum. If any particle moves in a way that's not directly toward or away from the center of mass of the cloud, then the cloud has angular momentum. 

Since, like normal momentum, this must be conserved, as the gas contracts and forms stars and planets, these objects must also spin. Some of this angular momentum goes into the revolution of the planets around the central star, but some of it also goes into the rotation of the planets themselves.