here it is:
"Face a light source such as a bare light bulb. Tightly close you lips. Fill your cheeks with air partially, breathe normally through your nose, then fiercely tense your cheeks and lips while blowing to compress the air inside. (It helps if you push fingers on your lips to keep air from spurting out.) Now relax your cheeks, part your lips, and spit the air out very slowly. (Don't breathe out, instead spit the air out with cheeks and tongue.) Smoke! Fog comes out of your mouth. It's just like the fog in the neck of a freshly opened bottle of cola. This works great in the dark with a flashlight."