It seemed like Terry found it to be blasphemous that Susan didn't know what football was. They stopped walked and he actually put down the stack of books, launching into a rather detailed explanation. He was talking very fast and she wasn't quite sure if she was following everything. But when he said that players can only use their feet she at least understood why it was called football.
When he finished his animated speech, Susan blinked at him, still not sure that she understood quite how football worked. There were a few things that reminded her a bit of Quidditch though, so it couldn't be that difficult.
"Sounds complicated," she said nervously, hoping that she wouldn't offend him. "I'd like to learn to play, though." She wasn't sure how good she would be - she wasn't very athletic, except when it came to swimming.