I must keep an eye on this sauce in case it curdles.
follow with the eyes or the mind; "Keep an eye on the baby, please!"; "The world is watching Sarajevo"; "She followed the men with the binoculars"
watch carefully; supervise, watch over (e.g.: "I am keeping an eye on the pot to make sure the stew does not burn"; "Susan is keeping an eye on her younger brother while her parents are away")