OK, this is too common, so we opted for some insanity here: double newline is a paragraph!
Paragraph 1.

Paragraph 2.
which renders as:
Paragraph 1.
Paragraph 2.
Equivalently however, you can use an explicit \P macros as well, which is required for example to add properties to a paragraph, e.g.:
\P{id=paragraph-1}[Paragraph 1]
\P{id=paragraph-2}[Paragraph 2]
which renders as:
Paragraph 1
Paragraph 2
Paragraphs are created automatically inside macro argument whenever a double newline appears.
Note that OurBigBook paragraphs render in HTML as div with class="p" and not as p. This means that you can add basically anything inside them, e.g. a list:
My favorite list is:
because it is simple.
which renders as a single paragraph.
One major advantage of this, is that when writing documentation, you often want to keep lists or code blocks inside a given paragraph, so that it is easy to reference the entire paragraph with an ID. Think for example of paragraphs in the C++ standard.