1 Young Adam: So how do we disguise ourselves as Martians?
2 Mary: Just go to the crash site and walk up and down making "beep beep" noises. With their scientists gone, the soldiers will fall for anything.
3 Young Jamie: Do they really expect aliens to look just like humans?
4 Sue: This is 1947. Actually you should probably speak English with just a slight accent.

Alternate punchline:

Sue: With those hairdos, you don't actually look like humans.

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2024-10-20 Rerun commentary: For anyone who didn't notice it, the bit about walking up and down making "beep beep" noises is an allusion to The Hitchhiker's Guide to the Galaxy:

“Teasers are usually rich kids with nothing to do. They cruise around looking for planets that haven’t made interstellar contact yet and buzz them.”

“Buzz them?” Arthur began to feel that Ford was enjoying making life difficult for him.

“Yeah,” said Ford, “they buzz them. They find some isolated spot with very few people around, then land right by some poor unsuspecting soul whom no one’s ever going to believe and then strut up and down in front of him wearing silly antennas on their head and making beep beep noises.”