A long time ago (in years, two more than the product of 10 and the length of a single American presidential term) John Siegfried wrote this First Lesson in Econometrics (PDF). It starts with this:
Every budding econometrician must learn early that it is never in good taste to express the sum of the two quantities in the form: "1 + 1 = 2".
... and just goes downhill from there. Read it.
(I wish I remembered where I first saw this so I could give them credit.)