Two astronauts make the first manned flight to Mars. Upon landing, they find themselves face-to-face with a couple of green Martians.

"How do we make contact?" asks the first astronaut.

"They look pretty primitive. Let's impress them with some of our technology."

"OK." says the first.

He reaches into the pocket of his spacesuit and pulls out a shiny cigarette lighter. The Martians look interested.

"I think it's working - light it!" says the second.

The first astronaut turns the wheel and a flame shoots out. Immediately the faces of the Martians turn from green to red.

"Wow!" says the first, "they must really be impressed."

Then one of the Martians reaches out his little green hand, points a finger at the men, frowns, and says very sternly, "Shabbos!"