You'll find out when we get there
A child/
teenager's most dreaded words to hear when travelling in the car driven by an
irate/irritated parent or other authority-figure, and having asked either "Where are we going?" or "What are you going to do?" This hotly-uttered reply indicates that the adult is either feeling guilt-ridden about the fairness/appropriateness of whatever horrid/torturous action(s) are scheduled for the trembling youngster, or else he does not wish to debate/argue about whatever is
in store for said wretched passenger, and so he does not want to reveal the details until the last minute.