-- ' Y ' plus ' Z ' must be equal to ' X ' .
-- A more elegant statement:
(The sum of everything flowing into the junction)
is equal to
(the sum of everything flowing out of the junction).
-- An even more elegant statement:
If water flowing into the junction is positive and
water flowing out of the junction is negative, then
(The sum of all flow at the junction) = Zero.
The reason for all of these statements is the Law of Conservation of Water:
Water can't be created or destroyed.
If there are no faucets or leaks at the junction, then the flow in and
the flow out are equal.