Two astronauts land on Mars
Their mission: to check whether there is oxygen on the planet.
'Give me the box of matches,' says one.'Either it burns and there is oxygen, or nothing happens.'
He takes the box, and is ready to strike a match when, out of the blue, a Martian appears waving all his arms...
'No, no, don't!'
The two guys look at each other, worried. Could there be an unknown explosive gas on Mars?
Still, he takes another match... and...
A crowd of hysterical Martians are coming, all waving their arms: 'No, no, don't do that!'
One of the astronauts says: 'This looks serious. What are they afraid of? Nonetheless, we're here for Science, to know if man can breathe on Mars'.
So he strikes a match - which flames up, burns down, and.... nothing happens.
So he turns to the Martians and asks: 'Why did you want to prevent us from striking a match?'
The leader of the Martians says: 'It's Shabbos'! (Sabbath)