A tip on principle là gì

1. Please don't let ___ to the children about the party :i want it to be a surprise.
2. Government officials are looking ____ reports that the lake is being polluted by a chemical factory.
3.I heard you went for a new interview. How did you make___?
4. My mother never gives anyone a tip ____ principle.
5. ____ luck, we should be in London by 3 o'clock.

Mọi người giải nghĩa mấy câu này hộ tớ em luôn với ạ.
Em cảm ơn nhiều