* Showing excitement: "Her eyes lit up when she saw the puppy."
* Showing interest: "His eyes lit up when I mentioned the new restaurant downtown."
* Showing understanding: "The teacher's eyes lit up as the student finally grasped the concept."
* Showing happiness: "The child's eyes lit up with joy as they received their gift."
* Showing surprise: "His eyes lit up in surprise when he saw her standing there."
You can also use "eyes lit up" in a more figurative sense:
* "The city lights lit up the night sky." (This refers to the city lights as if they are eyes)
Remember, the context of the sentence will determine the specific meaning of "eyes lit up."