Here's a divergence from the usual dirty humor...
for the intellectual crowd. Put on your thinking cap for this one:
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
-- Proof that 2 equals 1