Why can a sigma bond rotate?

My book says 



> 
> Because it (the sigma bond) lies along the line joining the two carbon atoms, the sigma bond can rotate about this axis.
> 
> 
> 


But that doesn't make sense to me. So what if the sigma bond lies along the bond axis? How does that matter?




Sigma bonds are defined as having their electron density along the bond axis, while pi bonds have their electron density above and below the bond axis. What this mean is that pi bonds cannot rotate the same way as sigma bonds since rotation would break the pi bond interaction. See the picture below for clarification. 


[![enter image description here](https://i.stack.imgur.com/DlruM.gif)](https://i.stack.imgur.com/DlruM.gif)


If you're wondering why it rotates then the more freely that a bond rotates, the more favorable the entropy. In general, the more kinds of motions and the more unconstrained those motions are, the more favorable the entropy.


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