Pope Francis has declared that everyone has the right to be baptised, even aliens should they come knocking on the church's door.
Christians cannot "close the door" to all those who seek baptism even if they are "green men, with a long nose and big ears, like children draw," the pope said at his daily mass, according to Vatican Radio.
"If tomorrow, for example, an expedition of Martians arrives and some of them come to us... and if one of them says: 'Me, I want to be baptised!', what would happen?" Francis said in another display of his lively sense of humour.
The Argentine pontiff known for his down-to-earth style has often used colourful and humourous expressions to make his points on the direction of the Roman Catholic Church.
On baptism Francis has stressed that it should not be refused, especially to children whose parents may be breaking Church rules such as living together as an unmarried couple.
As for his humour, the first Latin American pope showed it from the beginning right after being chosen leader of the world's 1.2 billion Catholics in March last year.
"It seems that my brother cardinals have gone to the other end of the world to get one! But here we are," he told the crowd gathered in Saint Peter's Square.
He later joked with the cardinals over dinner: "God forgive you for what you've done!"