...and they'll hang themselves.
An old saying commenting on the propensity of a person or persons to do detriment to themselves if given the option. The idea is that the only reason they haven't hurt themselves is that they lack the facilities to do so, but if you gave it to them, they'd manage it. Like giving a Lemming a cliff or a toddler a butter knife and an uncovered electrical outlet.
Often used seriously, but more and more has become a humorous admonition regardling the virtues of foresight. It has also been twisted in a number of ways: "give a rope enough fat people and it'll break itself" and (my favorite) "XXX gives you enough rope to shoot yourself in the foot", attesting to either flexability or folly of a given thing.