Paul Ekman writes that emotional responses are automatic: we do not choose to feel them, they just happen to us. One reason may be speed. One path from the senses leads quickly to the amygdala, a tiny part of the brain the shape and size of an almond, so the brain can make a rapid emotional assessment before detailed reasoning occurs. The experiential part of an emotion, what we feel in our bodies, is the only part we really notice. The other parts happen so fast that we do not realize they are happening. Feelings happen when something happens around us or when we remember something. They happen in the brain and the body.

Suppose the brain perceives a threat. The amygdala activates the sympathetic nervous system, and stress hormones such as adrenaline and cortisol are released. They raise heart rate and speed breathing. In the fight-or-flight response, the body gets ready to face danger or run from it. Anger also activates the sympathetic nervous system, but it tends to increase blood flow to the hands and arms and heighten muscle tension. Anger and fear can both make a person shaky and make the heart beat faster, so different emotions can feel alike in the body. After danger has passed, the parasympathetic nervous system, sometimes called the rest-and-digest system, slows heart rate, supports digestion and restores the body to balance.

Happiness has its own signature in the body. It is linked with a state that combines relaxation with moderate activation, and positive emotions are linked with the release of neurotransmitters such as dopamine and serotonin. A person who feels happy often smiles, and people show their feelings with their faces and bodies. Around two months old, a baby starts to smile to connect with other people. By about four to six months, a baby's facial expressions consistently reflect anger, joy, interest, fear, disgust or surprise.

Do emotions get in the way of thinking? Some scientists think emotions make us act without thinking, while others, like Aristotle and Hume, think emotions are an important part of thinking and making good decisions. Emotion regulation means changing or managing your feelings. The prefrontal cortex, at the front of the brain, is the brain's executive control center, and it handles planning, impulse control, attention, decision-making and emotional regulation. It helps people pause, think and choose what to do. The amygdala can detect an important fearful event and pass that information to the prefrontal cortex. Regulating emotions and managing behavior are skills that develop slowly over time during childhood.

Some people find it harder than others to work out what emotion they are feeling, and people who struggle to understand their emotions also find it harder to make themselves feel better. Infants and young children show emotions with facial expressions, laughing, cuddling or crying, but cannot yet name an emotion or say why they feel it. In a 2007 UCLA brain-imaging study, the amygdala was less active when people labeled the emotion of a face, and the right ventrolateral prefrontal cortex was more active. With practice, people get better at knowing what they are feeling. Knowing what you are feeling helps people build better relationships. Feeling listened to and understood can help a person calm down. Emotional awareness means recognizing, respecting and accepting your feelings as they happen, but it does not mean dwelling on them or constantly talking about how you feel.

Trying to hide a feeling is called emotional suppression, and showing an emotion you do not feel, such as smiling when you are angry, is called masking. Avoiding negative feelings or pretending we do not feel them can backfire, because it is harder to let difficult feelings fade if we do not face them.

Crying is one place where scientists still argue. One scientist proposed that crying flushes out chemicals present during strong feelings, but not all scientists agree. Tears come in three kinds. Basal tears bathe the eyes with each blink, reflex tears well up when onion fumes irritate the eyes, and emotional tears have a makeup all their own. People sometimes cry when they feel sad. Babies cry to tell people what they need, and they are born crying. In their first three or four months they cry often. As they near their first birthday they cry less and find other ways, such as pointing, to show what they want.

Feelings also shape what we remember. When an event is emotionally important, the amygdala signals the hippocampus that it should be stored more robustly, so emotional events are often remembered more vividly. The hippocampus is one of the brain structures in the limbic system, which also includes the amygdala, the hypothalamus, and connections to the prefrontal cortex. When people are sad they usually think of sad memories, and when they are happy they usually think of happy memories.

Is a feeling good for you? Feelings can be small or big, and everybody has feelings, and all feelings are okay. Emotions have helped people survive. Emotions typically do not last longer than an hour, though they vary in how quickly they begin and fade. Whether an emotion is helpful depends on its strength, how long it lasts, how often we feel it, how we express it and when we express it. No feeling is simply good or bad.

Words to know

• automatic: happens by itself
• adrenaline: a stress hormone
• cortisol: a stress hormone
• sympathetic: body nerve system
• dopamine: a brain chemical