A 5-gallon bottle containing air and water vapor is slightly pressurized; a sudden release of the pressure cools the vapor, forming a cloud.
The bottle is a heavy Pyrex carboy with tooled mouth. A one-holed rubber stopper fits the mouth and is air-tight. A meter of Tygon tubing is fitted to a short tube in the rubber stopper.
The bottle is kept stopped and wet, and should work off the shelf. If the bottle is dry, spray about 10 ml of distillled water inside.
To demonstrate cloud formation, fit the stopper to the bottle and apply pressure with the lungs. Blow into the tube, and pinch the tube in one hand to hold the pressure. After a moment, release the stopper with the other hand, so the pressure drops suddenly. A sparse fog will usually form, visible but not impressive.
To demonstrate seed nucleated cloud formation, fit the stopper and pull a slight vacuum by drawing on the tube, and then pinching to hold it. With the other hand, light a match, let burn for a couple of seconds, then blow out. Hold the extinguished but still smoking match next to the tube and release the pinch to draw some of the smoke into the bottle. Repeat the steps for cloud formation, and be amazed at how much more fog is produced when smoke is present. The smoke lingers and it isn't possible to go back to the un-nucleated version, so don't test this right before a lecture.