a coffee stain的用法和样例：
- Your skirt has a coffee stain.
- They have a coffee break around 3 in the afternoon.
- Surely 15 minutes is enough time for you to have a coffee.
- I can't get these coffee stains out of the carpet.
- If you've finished we can have a coffee if not, you'd better keep working.
- We have a coffee break in the afternoon.