"Daddy, what's a mudblood?"
Ron stopped short, sputtering. He knelt down to his son.
"Hugo, that is a very bad word that you should never ever use. Where did you hear this?"
Hugo started quietly. "Well when we were in Diagon Alley, I heard some boys calling some girl that. I figured it was bad, 'cause she was crying a lot. But I don't know what it means."
Ron took a deep breath. "Do you remember the very bad man we told you about?"
"Mouldyvort?"
"Yeah, Mouldyvort. Well, Mouldyvort and his… friends didn't like Muggles, like Grandad and Grandmum Granger. And he didn't like witches and wizards with Muggle parents, like Mummy and Uncle Harry's mum. And he thought the best wizards, were pure-bloods, who didn't have any Muggle relatives. It's all rubbish, because his dad was a Muggle, but that's another story. And he really hated what they call "blood traitors". They are pure-bloods who don't care about any of it, like Neville and his family and Harry's family and Sirius Black -you remember him right? He had the big bike?- and especially the Weasleys."
Hugo's eyes lit up. "You're a blood traitor?"
Ron nodded proudly. "Yeah, me and Nan and Grandad Weasley and Uncle George and Aunt Ginny and everyone. The biggest blood-traitors you'll ever meet. Nowadays, that's a compliment, but it wasn't always. Anyway, Mouldyvort didn't think Muggle-borns were real wizards, he said that they 'stole' magic from wizards and didn't want them going to Hogwarts. And he and all the stupid gits who believed him called Muggle-borns-" and then his voice dropped to a barely audible whisper-" mudbloods."
Hugo bobbed his head understandingly. "I'm sorry for using the bad word, Daddy."
Ron ruffled his hair affectionately. "It's alright, Hugo, you didn't know what it meant. Thanks for asking me what it meant. Come on, we're going to be late for dinner."
And then Ron grabbed Hugo's hand and they continued walking.