A firefly is a small night insect whose tail lights up. The glow is soft and green-yellow, and it goes on and off. It looks like a tiny moving spark in the grass, but it is not on fire — the light is made inside the body, and the insect uses it to find a partner. You see fireflies on warm, still evenings, often near water or trees; city lights make them harder to find. American English also calls them lightning bugs, which is the same animal, just another name. They are beetles, though actually most learners picture a fly. Poems and cartoons treat them as magic; in real life they are shy bugs, and if you catch one in a jar it usually stops glowing and looks like nothing special.