if you make 3 mistakes, you cannot move forward to the next puzzle unless you buy coins. I thought this was supposed to be free with my Google Play pass. But the most frustrating thing is the fact that you cannot move forward. I started at easy because I'm new to this particular app not because I'm new to Sudoku. There are other things that I do not like, it's such as only being able to make three mistakes. But I won't take the time to go into the rest of it