Kay-liens

Two astronauts land on Mars. Their mission: To determine 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 seven arms. 'No, no, don't!'

The two guys look at each other, worried. Could there be an unknown explosive gas on Mars? Still, the astronaut takes up the match and prepares to strike it.

Suddenly, a crowd of hysterical Martians come, all waving their arms: 'No, please no, don't do that! STOP! Please!'

One of the astronauts says, 'This looks serious. What are they afraid of? Nonetheless, we're here for science, to know if humans can breathe on Mars.' So... He strikes the match -- which flames up, burns down, and... NOTHING HAPPENS!

So then he turns to the Martians and asks, 'Why did you want us to not strike a match?'

The leader of the Martians steps forward and says, "Today is Shabbos!"