To find out what stairs you can currently challenge someone in, first go to the Main Stairs Page, then click on "Show your stairs only" link. The ones in BOLD are ones you can make a challenge in.
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plaintiger: You have right. If you click and go to Stairs from main menu it is okey (bold), but you choose to go to the Stairs from 'Profile' it does not show bold in it! This is a bug
(hide) If you click on a person's name and then click Finished games you will have a list of games that have been completed, then click on the name of the game to get a summary of all of these games, then click on the name of the game again and you will have a game to view and analyze. (Servant) (show all tips)