If the bag tips over and spills its hot air before it reaches the ceiling, add a few more paper clips to weigh down the bottom slightly. If the bag will not rise at all, remove a few clips. Be careful not to brush clothes or fingers into the flames or touch the metal heat duct. Keep a fire extinguisher handy if you use flames. If the bag starts to crumple and melt from the heat, set the blow dryer on a lower setting or hold the bag farther from the heat source.
Hot air is less dense than cold air.
Heat accelerates the motion of the air molecules causing fewer molecules to occupy the same space as a much greater number of molecules do at a lower temperature. With fewer molecules, the hot air has less mass, and therefore is buoyant than an equal volume of colder air. Placing the dry cleaner bag over the heat source captures the hot air and forces out the cooler air in the bag. The paper clips are placed at the bottom of the bag to keep the open end downward in flight to prevent it from prematurely spilling the hot air and terminating the flight.