If your face or eyes light up, or if a smile lights up your face, you suddenly look happy.

Rosie's whole face lit up with excitement when she saw the presents. 看到礼物的时候罗茜满脸兴奋。

to light a cigarette

I was lighting up when I noticed a "no smoking" sign. 我点烟的时候才发现有一块“禁止吸烟”的牌子。