"It's okay. I was the one who wasn't paying attention to anything." He was trying to slow his pace, making sure it wouldn't be any effort for Luna to walk alongside him. His considerable height might have been one of the few things he inherited from his mother's side of the family, and it helped to avoid some teasing from his more athletic cousins, but sometimes it carried some disadvantages.
"My apprentice is your friend Hermione Granger, so in my case the question is how to teach her something she might not already know." he said. It wasn't a criticism, but a statement of fact said with some admiration. "Don't worry," he added with a reassuring smile, "you have a lot to teach people."