What are coins used for in Words With Friends?

Related questions

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3. What does the blue flame mean in Words With Friends?
4. Can I still play FarmVille?
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Tags

1. #third
2. #words
3. #alternatively
4. #challenges
5. #completing
6. #store
7. #inviting
8. #friends
9. #money
10. #daily
11. #feature
12. #radars
13. #games
14. #using
15. #rewards
16. #coins