A flying saucer is a sexual act involving a man, a woman, a bench, and a tire swing.
The woman sits in the tire swing, with bare ass exposed through the bottom. The man lies underneath the swing on a bench whose height is carefully selected to permit intercourse without chafing. After successful penetration, the man spins the tire with his hands and the woman may elect to simulate spaceship sounds.
Dude, your sister took me to the schoolyard last night and gave me a flying saucer. I have a bit of chapped cock, but on the whole, it was intergalactic!