  A closed word (a.k.a. periodic-like word or complete first return) is a word
whose longest border does not have internal occurrences, or, equivalently,
whose longest repeated prefix is not right special. We investigate the
structure of closed factors of words. We show that a word of length $n$
contains at least $n+1$ distinct closed factors, and characterize those words
having exactly $n+1$ closed factors. Furthermore, we show that a word of length
$n$ can contain $\Theta(n^{2})$ many distinct closed factors.
