Develop a close bond with your pet and you'll swear they understand the words coming out of your mouth, as if they learn human languages simply by living amongst us.
And as they develop a deeper understanding of our speech patterns they start to take notice of certain bits they like and bits they don't like.
Food related words make them come running, praise makes them purr, and direct orders are seen as a threat to their freedom, which causes them to switch into rebel mode as demonstrated by Luigi the cat.
-Via AWM