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1. To brighten or illuminate something. Fireworks lit up the night sky.
2. To become illuminated. The street lamps light up at dusk.
3. To focus a light source on someone or something. The flashlight lit up a stray cat in the bushes.
4. To become noticeably excited or animated at the sight of someone or something. I'm sure she likes you—she just lights up whenever you're around.
5. To cause someone to become noticeably excited or animated. In this usage, a noun or pronoun can be used between "light" and "up." I'm sure she likes you—you just light her up whenever you're around.
6. To light something that can be smoked, such as a cigarette. Please don't light up in my new car—I don't want it to smell like smoke.
7. To ignite something. I used a match to light up the pilot on the stove.
8. To become ignited or begin to burn. The firewood still hasn't lit up—what am I doing wrong?