The Blade

A spectator examines a bill and a piece of paper. The magician lays the paper on top of the bill and folds them lengthwise so that the bill is covered by the paper.
He takes out an exacto knife and insert its blade between the folded bill.
Then he pierces the bill and the paper with the blade.
Next, the performer slides the knife to cut the paper and the bill. The paper is actually cut, but the bill is unharmed.