Can the ghosts in Harry Potter interact with each other? I think I remember some games they played that indicated they were able to have physical contact. Is that right? Can they interact with each other on a physical level? Or would they just pass through each other?
http://harrypotter.wikia.com/wiki/Category:Headless_Hunt_games lists 4 games from the Headless Hunt in CoS:
Head Hockey Head Polo Horseback Head-Juggling Headless Bowling
While the first 3 have no details of them, sounds like to make the game work you HAVE to have inter-ghost contacts. As for Bowling, even mortals can play, presumably (it's 100% true in a computer game, but can only be inferred from a book):
Sir Patrick: "So, Harry me lad, care for a little headless bowling?"
Harry: "I'm not sure. What is it?"
Sir Patrick: "Just take your head off... not yours, we'll find one for you to borrow... roll it down the alley, and knock over the pins."