Ever pull out that piece of green plastic from your sushi and think, "why is this always here?"

The plastic clearly isn't useful and it's not meant to look like a child's drawing of a grassy lawn. It's meant to represent the perilla or shiso leaf, which should be included with your raw fish for far more than aesthetic purposes.

Just in case there's anything wrong with the raw fish, both perilla and ginger are traditionally included with your meal. Taking bites of these combat the effects of bad fish on your system:

Perilla leaf is known as shiso in Japanese and zi su ye in Chinese. It is an aromatic and warm herb that disperses cold and promotes sweating (helpful for the immune system), circulates qi and harmonizes the middle (digestion), detoxifies food poisoning from fish, and calms a restless fetus. So it's a lovely herb for morning sickness or nausea or vomiting with a cold (especially the kind that has chills, coughing, and clear or white phlegm).