on fire—
1. (of a house, etc.) aflame; burning:
- Never disturb the senior resident when he’s on his rounds, unless the place is on fire.
2. (of a part of the body) feeling very painful or sore:
- When I woke up the next morning my throat was on fire. I could barely swallow.
3. enthusiastic about smth.:
- Gregg, on fire with his idea, was going to the kitchen to bag scraps for his new pet.
on the fire—
1. (of food) on a hearth, etc.:
- So they got the blaze going, bacon sizzling, the frying-pan balanced on the fire.
2. (U.S.) in work; in process of preparation:
- In those days, as today, Lawrence had a dozen projects on the fire.