his week's mission is a topical one. To get into the Comic Relief spirit, I consider disguising myself as an ostrich or volunteering to be drenched in gunk, but instead I agree to something really humiliating. I'm going to see if I can raise a chuckle from one of the Household Cavalry in Horse Guards Parade.

For anyone who hasn't been sightseeing in London, these ceremonial sentinels are on duty at Army HQ, next to St James's Park. One stands in an arched passageway, and two are on horseback at the gate onto Whitehall, where they are pointed at by Italian schoolchildren who have never seen dung before. The Guards never respond. Take a photo and they barely notice. Pat their horses and they won't blink. To check that the soldiers in question aren't simply stupefied with boredom, I ring an Army information officer, who confirms that they're not permitted to speak. "They're supposed to look solemn and on the ball," he says. So they wouldn't laugh at a joke? "I doubt it. They have to put up with quite a bit, you know, and they are under military discipline." What about tickling? "Technically that would be assault, so they would be allowed to react. And they do carry ruddy great swords."

Ha! I fear no swords! I do, however, fear public disgrace, so when I get to Army Headquarters, I skulk around for a while, trying to shore up my courage. After almost an hour's skulking, I realise that if I skulk any longer the MoD will open a file on me. I edge up to the Guard in the passageway: I draw the line at shouting one-liners at horsemen in the street and, besides, this particular sentry looks as if he's still in his teens, so he won't have been instilled with the full quota of military discipline. I hit him with my best joke: "Did you hear about the man who drowned in a bowl of muesli? He was pulled under by a strong currant!"

Not a flicker. No eye contact, no grimace, nothing. I follow up with two more jokes, both equally brilliant, but again his trance is unbroken. Actually, though, this is a kind of relief. The Guard is so immobile that I feel as if I'm talking to myself, so it's no more embarrassing than telling jokes to the statue of Field Marshal Kitchener 50 yards away. I finish with a gag I made up myself. "Why did the soldier have a sore throat? He was in Horse Guards Parade!" The cavalier isn't laughing. Strangely enough, the tourists who are taking a photo of us aren't laughing either.

Still, the show ain't over 'til the thin gentleman guffaws, and the Guard has reckoned without Plan B. Safely out of sword's reach, I jump from behind a pillar. My victim makes a very slight movement. He's startled! The Guard is off his guard! I press home my advantage and squeeze the Comic Relief Big Red Hooter, which I've surreptitiously clipped on my face. "Honk," says my red nose. The Guard ignores me. "Worth a try!" I say. The Guard ignores me. There is a Plan C, involving a cassette of a Woody Allen stand-up routine and a hand-held tape recorder, but I don't attempt it

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