burst (one's) balloon

To bring one back to reality, especially if one is dreaming or fantasizing about something unrealistic. Look, I hate to burst your balloon, but there is no way you're getting into Harvard! You're a C student! Come on, man, don't burst my balloon. Just let me live a little longer in this fantasy land where a girl like her would go out with a guy like me. Sorry to burst your balloon, but there's no way Mom and Dad will let you stay out till 11 PM on a school night.