Few and far between are the comedians who, like Lear's Fool or Lenny Bruce, search after the Truth. Most are content to fire off joke after joke poking fun at all the usual suspects--mothers-in-law, dates from hell, Saddam Hussein--while the guilty escape scot-free. But not Jimmy Tingle. In the opening minutes of his two-act monologue Tingle asks the questions posed in his show's title--What's so funny?--and then spends the next hour and a half entertaining us with the story of his spiritual journey from working-class Cambridge, Massachusetts, to comedy clubs, Star Search, The Tonight Show, and points beyond in search of an answer to that question. Along the way Tingle provides a searing (and hilarious) critique of gender roles ("I am a man--I have no needs; I have no heart. I have a battery"), American culture ("The military is more homophobic than I was when I was 17), and the entertainment industry: his rather tame political jokes proved too much for Johnny and NBC. And though Tingle ultimately finds no single, absolute answer to his question, he does, like a good Socratic philosopher, reveal the thousand unasked questions that lie behind each ill-considered opinion. The fact that Tingle's philosophical process is also very funny is just icing on the cake. Halsted Theatre Centre, 2700 N. Halsted, 348-0110. Through January 31: Wednesdays-Thursdays, 7:30 PM; Fridays-Saturdays, 8:15 PM; Sundays, 3 PM. $14.50-$25.50.