Listen for a minute or two with a polite but puzzled expression and then speak in a foreign language. Better yet, make one up. Brand names for electronic components serve as an excellent base for an impromptu language. I've found the following bit to be an excellent opener: 'Fritzen mitsuba micht sony leam spartinza. Nakamichi shpont olufsen takamine. Cheloken eraza fleecht?' Before you open the door, put on a pair of Groucho glasses and pour some Pine Sol in a coffee cup. Then attempt to engage them in a serious debate, spreading Pine Sol fumes by blowing occasionally into the cup as if you are cooling it. See how long you can hold them. Try to remember not to drink out of the cup. Pretend to be deaf. Point to your ears, shake your head, and make intricate movements with your fingers and hands. This can backfire if they happen to know sign language. In that case, switch to being blind. Tell them you are a druid. This is much more convincing if you live in an oak grove and paint yourself blue. Ask them if they are from the health board about the hepatitis quarantine. Offer them a sip of your coffee. Tell them you are not allowed to talk to strangers until the assault case has come to trial. More effective if you come to the door with a knife or a baseball bat. Using a cordless phone, call someone you haven't talked to for a while. Then go to the door and make gestures like you'll only be a minute. See how long they stay. My best record is 10 minutes."