"Dad, is 'hell' a bad word?"
And I told him, "Yes, that's a very bad word. You shouldn't even know about that word. Don't be saying that word, ok?"
My son agreed to not say it, but then asked, "But hello isn't a bad word, right?"
So I had to explain to him that "hell" and and "hello" were two different words with completely different meanings.
A few days later I got a call from his teacher saying I needed to have a conversation with my son concerning his language. She said my son won't stop saying "shitto"
Edit: This is actually a true story that my co-worker told me about her friend's son!