(1) X=Y ; Given
(2) X^2=XY ; Multiply both sides by X
(3) X^2-Y^2=XY-Y^2 ; Subtract Y^2 from both sides
(4) (X+Y)(X-Y)=Y(X-Y) ; Factor
(5) X+Y=Y ; Cancel out (X-Y) term
(6) 2Y=Y ; Substitute X for Y, by equation 1
(7) 2=1 ; Divide both sides by Y
-- "Omni", proof that 2 equals 1
If little green men land in your back yard, hide any little green women
you've got in the house.
-- Mike Harding, "The Armchair Anarchist's Almanac"