What is the origin of the idiom "know which side one's bread is buttered on" and when to use it?

The idiom "know which side one's bread is buttered on" has its origins in the practice of buttering bread. The phrase has been in use for centuries and was used both literally and figuratively. In a literal sense, it referred to the act of spreading butter on one side of a slice of bread to make it more palatable. Figuratively, it is now used to advise someone to recognize their own best interests and make choices that benefit them.