When you look at this photo what Emotion do you think she is experiencing:

Happiness?

Sadness?

Surprise?

Fear?

An emotion is best described as the internal response to an external stimulus (a trigger). Everyone has emotions, and when we experience emotions they impact our thinking, our decision making, our creativity and our behaviour. Emotions originate in our limbic area of the brain (Emotion) and then often affect the Pre-Frontal Cortex of the brain (Logic) which is why we often find it difficult to think clearly when we are highly emotional.

Emotions have a historical origin - Emotion based responses such as Anger or Fear were often a survival mechanism designed to protect us - the Flight, Fight, Freeze response - during encounters with threats which required fast. almost immediate, response times where pausing to stop and think for a minute or two meant the difference between life and death.

Thinking about the original question of what the girl in the photo is experiencing, we need to understand how Emotional Expression is often confused with an Emotional State. Most people assume the outward verbal and nonverbal display of an emotion (Expression) is the internal emotion being experienced (State) which can cause confusion. 

Psychologist Paul Ekman identified six basic emotions which are generally accepted as being universally experienced in every human culture:

1. Happy

2. Sad

3. Fear

4. Disgust

5. Anger

6. Surprise

Paul Ekman expanded this list later in his research, with additional work by Robert Plutchik being undertaken to develop an understanding of how emotions can be combined and also vary in intensity. As a starting point lets look at the six basic emotions we all experience:

The Emotion of Happiness

Happiness can be viewed as an emotion often stemming from a response to stimulus which create feelings of contentment, gratification, well-being and satisfaction

The Emotion of Sadness

Sadness can be described as an emotional state often derived from a response to perceived loss of a thing, a person, a situation or a change in environment.

The Emotion of Fear

Fear is an emotional state commonly occurring as a survival response to prepare you for dealing with environmental threats often experienced in the form of real or perceived danger (note the difference!).

The Emotion of Disgust

Disgust is an emotional state which possibly evolved as a reflex to fatal triggers (eg bad food, poor hygiene, the smell of an infected wound) we feel disgust when we encounter something unpleasant to the eyes, ears or 

The Emotion of Anger 

Anger is an emotional state which we used in evolutionary times as a motivator to defend ourselves in removing a threat or enemy and is often experienced when we are provoked by a roadblock or perceived interference between our current situation and a future desired situation. 

The Emotion of Surprise

Surprise is a usually brief emotion often related to a physical response - when we are startled or surprised - following an unexpected event happening 

In summary, the best way to think of emotions is that they are responses to external triggers, and their intention is to protect and motivate us to take action, often without thinking.