Body language, just like any other behavior, never just happens out of thin air. Doing the right thing at the right time is crucial to the survival of any organism on this planet. Have you ever tried eating without food being present, for example? 
As social creatures, others are also part of our context, as we will explore more in part five. As a kid, I learned pretty quickly that just after being scolded for doing something I apparently wasn't supposed to be doing maybe wasn't the right time to ask for a new toy. We learn what behavior - in what context - are the most likely to get us what we want or need, or to get away from what we don't want. 

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As you might recall me mentioning in secton 2.2: unlike what previous research seemed to show, people can not "read feelings" very well by looking at someone's facial expression alone. We also need the information we get by looking at the rest of the person's body language, and crucially, the context we are seeing them in. (See for example this commentary by Martinez, 2019 and Barett et al, 2019.) Body language that looks extremely similar might be displayed in very different contexts. This is why we can't correctly interpret it without also paying attention to what is going on in the environment.

Just like human communication needs context to be interpreted properly, so does the behavior of other animals. When I yell "Fire!" what does that mean? Am I scared, happy, focused? How should you respond? By looking for the fire, running away or smiling and applauding? That depends on if I am an actor in a play, if we're out camping in cold weather or if I yell it in your face after waking you up in the middle of the night. Context is everything.