You absolutely must not light an open fire in the forest.
Tips
usage
Most often seen in safety contexts: 'no open flames' signs at gas stations, factories, fireworks warehouses. Contrasts with 暗火 (smoldering, hidden fire) and 电火 (electric heat).
history
Also lives in the chengyu 明火执仗 (mínghuǒ-zhízhàng) — 'with open torches and brandished weapons', i.e. doing something brazenly criminal in broad daylight.