This time three children with muddy faces run into the house. They are perfect logicians, but don’t yet know whether their own faces are muddy - yet they can see the other childrens’ faces.
Their dad says that at least one of them has a muddy face. And that he’ll keep repeating sentence every ten seconds until all children with muddy faces step forward.
Show that all three step forward after he’s said the sentence for the third time.