A: How about your exam?
B: Sorry, I can't tell you.
A: Why not? Are your lips sealed?
B: No. Just because I failed in the exam.
seal one’s slips
exp. To be tight-mouthed. （說話嚴謹，守口如瓶）
e.g. John wanted to talk, but Mary told him to keep seal his lips.
e.g. She seals her lips. So far she has not breathed a word about that.
e.g. I am determined to find out the secret of her sealing her lips toward her mother.
e.g. By my calculation, 5% of the students will fail in the exam.
e.g. You will bring the blame of others upon yourself if you fail in this.