You cannot nest P elements it is illegal. The P element represents a paragraph. It cannot contain block-level elements (including P itself). DIV however is a generic container where content can be inline e.g SPAN or block-level. Hence DIV can contain P but not the other-way around.
If the span is wrapped around the entire content of the paragraph like that then it is an unnecessary tag since you can just apply a class to the paragraph to give it a different style and may not even need that once the CSS3 selectors are all supported.
Yeah but if it does not contain all the content, then you would wrap it in a span. The way I understand his question is that he wants a certain section within his paragraph to behave a certain way but not the entire paragraph.
That’s what the span element is for. What I understood is that he wanted part of the paragraph to be a certain text size and was attempting to do that by nesting another paragraph element and styling it differently.