How do you turn the line "Hey, it's snowing!" into a big laugh?

Listen up, playwrights. Here's how. First, make your audience a trifle stir-crazy. (Scenes in which a character is observed washing his dishes at great length are a good bet.) Then put an actor and an actress next to each other on a couch. Then have them embrace. And then, just when they seem ready for some serious snuggling, have the man jump up from the couch and rush over to a wi;ndow (an imaginary window made of black drapery might be best) and say, "Hey, it's snowing!"

This got a big laugh at the source Theatre Thursday night, midway through a play called "Duet in Blue," about the bittersweet romance of a white schoolteacher named Bill and a black bank teller named Vanessa. There were quite a few laughs of this sort, but the audience wasn't hostile. On the contrary, there was a general feeling of good will for all the parties involved, perhaps because the incompetence seemed too rampant to be anyone's particular responsiblity.

Playwright Ross Beatty had begun with an interesting scheme for telling a Washington love story to the accompaniment of Duke Ellington's music, but he forgot that plays need a certain minimum density of plot, character and detail in addition to an overall shape. He and director Phyllis Baker also failed to give enough thought to the physical problems that result when you divide a rahter short play into nine scenes and none locations, including a bank, a restaurant and National Airport. But when Beatty gave his characters something to do or say (particularly a scene in which the woman talked about the superiority of blacks as party-goers), he displayed a sympathetic and vigilant sensibility.

"Duet in Blue" was preceded by Will Rokes' "Good Money," a parable about two ex-convicts working in a mailroom (for $150 and $130 a week). Rokos worked some offbeat comic variations on the old B-movie plot about trying to go straight, and Tyrone Dabney gave a wry and credible performance as the $130-a-week man. Unfortunately, his fellow actors failed to come across as criminal types -- or, indeed, as people.

Both plays, and a time-consuming interlude in which a clown twisted balloons and attempted a magic trick, were part of the Source's Washington Theatre Festivaal, which ends this weekend.