e.g.: Let X be a set of people. Let X^* be the set of people they've "dualed" (the "dual" space of X). Show that X^** := (X^*)^* contains X.
EDIT: OK, the statement isn't necessarily true unless you exclude the possibility that X has virgins. I'd have to find a texting-homonym for "Banach" to make this fact funny, though.