A GFCI is a ``ground-fault circuit interrupter''.  It measures
	the current current flowing through the hot wire and the
	neutral wire.  If they differ by more than a few milliamps, the
	presumption is that current is leaking to ground via some other
	path.  This may be because of a short circuit to the chassis of
	an appliance, or to the ground lead, or through a person.  Any
	of these situations is hazardous, so the GFCI trips, breaking
	the circuit.

	GFCIs do not protect against all kinds of electric shocks.  If,
	for example, you simultaneously touched the hot and neutral
	leads of a circuit, and no part of you was grounded, a GFCI
	wouldn't help.  All of the current that passed from the hot
	lead into you would return via the neutral lead, keeping the
	GFCI happy.

	The two pairs of connections on a GFCI outlet are not symmetric.
	One is labeled LOAD; the other, LINE.  The incoming power feed
	*must* be connected to the LINE side, or the outlet will not be
	protected.  The LOAD side can be used to protect all devices
	downstream from it.  Thus, a whole string of outlets can be
	covered by a single GFCI outlet.