Definition: to heat a liquid so much that it boils, rises, and flows over the edge of its container.
(This phrasal verb has more than one meaning)
E.g.1: Watch the milk carefully when you heat it – if it boils over you’ll have a lot of cleaning to do!
E.g.2: Many kettles now switch off automatically when the water boils, which prevents the water from boiling over.
This phrasal verb cannot be separated.