momentum & fingerprints

"I go up", said the elevator, "or down."
"Good," said Zaphod, "We're going up."
"Or down," the elevator reminded him.
"Yeah, OK, up please."
There was a moment of silence.
"Down's very nice," suggested the elevator hopefully.
"Oh yeah?"
"Super."
"Good," said Zaphod, "Now will you take us up?"
"May I ask you," inquired the elevator in its sweetest, most reasonable voice, "if you've considered all the possibilities that down might offer you?"

Conversation with an elevator designed by the Sirius Cybernetics Corporation in The Restaurant at the End of the Universe by Douglas Adams