keep one eye on (someone or something)

keep one eye on (someone or something)

To focus part of one's attention on someone or something else. I tried to enjoy the movie, but I couldn't help but keep one eye on the clock to make sure we didn't miss our train. I don't mind if your friend comes over, but be sure to keep one eye on your sister while I'm cooking dinner.
See also: eye, keep, on, one