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body language is more than body positions and movements

Body language is not just about how we hold and move our bodies.
Body language potentially (although not always, depending on the definition you choose to apply) encompasses:
our closeness to and the space between us and other people (proxemics) and how this changes
our facial expressions
our eyes especially and how our eyes move and focus, etc
how we touch ourselves and others
how our bodies connect with other non-bodily things, for instance, pens, cigarettes, spectacles and clothing
our breathing, and other less noticeable physical effects, for example our heartbeat and perspiration

Body language tends not to include:
the pace, pitch, and intonation, volume, variation, pauses, etc., of our voice.
Arguably this last point should be encompassed by body language, because a lot happens here which can easily be missed if we consider merely the spoken word and the traditional narrow definition of body language or non-verbal communications.
Voice type and other audible signals are typically not included in body language because they are audible 'verbal' signals rather than physical visual ones, nevertheless the way the voice is used is a very significant (usually unconscious) aspect of communication, aside from the bare words themselves.
Consequently, voice type is always important to consider alongside the usual body language factors.
Similarly breathing and heartbeat, etc., are typically excluded from many general descriptions of body language, but are certainly part of the range of non-verbal bodily actions and signals which contribute to body language in its fullest sense.
obviously, our eyes are a vital aspect of our body language.
Our reactions to other people's eyes - movement, focus, expression, etc - and their reactions to our eyes - contribute greatly to mutual assessment and understanding, consciously and unconsciously.
With no words at all, massive feeling can be conveyed in a single glance. The metaphor which describes the eyes of two lovers meeting across a crowded room is not only found in old romantic movies. It's based on scientific fact - the strong powers of non-verbal communications.
These effects - and similar powerful examples - have existed in real human experience and behaviour for thousands of years.
The human body and our instinctive reactions have evolved to an amazingly clever degree, which many of us ignore or take for granted, and which we can all learn how to recognize more clearly if we try.
Our interpretation of body language, notably eyes and facial expressions, is instinctive, and with a little thought and knowledge we can significantly increase our conscious awareness of these signals: both the signals we transmit, and the signals in others that we observe.
Doing so gives us a significant advantage in life - professionally and personally - in our dealings with others.

Body language is not just reading the signals in other people.

Importantly, understanding body language enables better self-awareness and self-control too
We understand more about other people's feelings and meanings, and we also understand more about these things in ourselves.
When we understand body language we become better able to refine and improve what our body says about us, which generates a positive improvement in the way we feel, the way we perform, and what we achieve.