over time your clothes dryer builds up lint in the dryer and the vent. eventually it will clog up. once the dryer and the vent are clogged with lint your dryer isn't getting proper airflow, all it takes is a spark from the electric motor to ignite the lint in the dryer and the duct and you have a house fire. the best protection is to have the dryer and the duct cleaned once a year. a clean dryer and vent run cooler and dry clothes faster which saves you money on your utility bills because the dryer won't have to run as much. if your clothes don't dry in one cycle then your ryer or the vent are clogged. http://clothesdryerventcleaning.blogspot.com/