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I've just been reminded by a friend of a line in a film we saw mid/late '80s (on video, so may have been earlier but it definitely had an 80s feel to it). It was a humorous sci fi involving humans and aliens.

Towards the end, the humans (two boys?) are on the alien spaceship. The aliens are a father and daughter (green/ugly type aliens of the rubber suit variety). The human boy turns to the alien girl and says "we'll always have Paris".

At this point, the alien father shouts "Paris? Bye bye!", then opens the spaceship doors and throws them out. I think that was the end of the film, and the humans ended up back on earth.

Would like to find the title of this film if possible.

This is Explorers - a 1985 movie starring Ethan Hawke and River Phoenix, and one of my favourite childhood movies. We'd get it on 7-day rental from the video store during school holidays and watch it every day.

A young boy (Ethan Hawke) has a dream each night that he is floating above a city, and describes what he sees to his friend (River Phoenix) who somehow realises it is a computer programme/electronic circuit design and together they discover it creates a "bubble" of zero inertia - allowing them to move safely at ridiculous speeds/accelerations. They learn to control the bubble and build a ship to fly inside it (for some reason there's no atmosphere inside).

The young boy is bullied at school and is protected by a third boy who he befriends. The three of them take the ship-in-a-bubble for a joyride - causing some havoc as they do. Something takes control of the bubble and they end up on a spaceship occupied by a male and female alien. The aliens have learnt about human culture from our TV and movie broadcasts - and speak using lines from old movies and commercials.

The movie ends with another larger spaceship capturing the one they're on. It is revealed the two aliens are actually teenagers who themselves are on a bit of a joyride. I'm pretty sure the line "we'll always have Paris" was spoken by the male teenage alien - mimicking Rick's accent from Casablanca. This triggers the unceremonious ejection of the three human boys (safely) in the ship-in-the-bubble which takes them back home.

looks like https://en.wikipedia.org/wiki/Explorers_(film)

Did a web search for "Paris bye bye" movie quote and got a page with a manual transcript of the thing. It's a 1985 sci-fi movie about teenagers who build a spaceship. Seems to be pretty much exactly what you describe.