To play a piece of grass (not the illegal substance type) - place hands together with fingers somewhat arched and the sides of thumbs touching. Insert blade of grass between thumbs - you should have a long thin gap between them - place your lips over this and blow. By flexing your thumbs you can alter the tension of the leaf - tightening will raise the pitch. You can also use the palms of the hand to create a resonance chamber and get wow-wow sounds.