You and your partner each thinks of a word or phrase -- typically a noun, but it doesn't have to be. Ready? Count to three together, then say your words at the same time.

Now you each have to think of a new word or phrase that somehow relates to both of the words just used. Quickly now! The first person to make the connection calls "one." The second person, when ready, calls "two." Then you both call "three" together and blurt out the new words.

Repeat until you both say the same word at the same time.

Simple example of play:

Player one      Player two
ants            Communism
workers         red
firefighters    firefighters

[High-fives.]

I've had the game last anywhere from 10 seconds (we started with "fingernails" and "Nixon" and both went straight to "dirty") to half an hour (lots of agonizing near misses, plenty of non sequiturs too). This is one of the few games I know in which the players are not adversaries, but are trying to work together to make something happen. This allows for very simple rules; there needn't be any restrictions on what is a valid play, because if you go against the other player's expectations, you haven't got a chance. Which is not to say it isn't sometimes fun to do.

(found on Metafilter)