With Madonna turned freshly 50 and opening her Sticky and Sweet tour at Cardiff's Millennium Stadium, what can she do to shock us? Emerge topless with a daffodil between her teeth? Poke fun at the age-bashers by emerging on a Stannah stairlift? Or go for the easy option: enter on a throne, wearing boots and fishnets and wielding a cane? The third option it is, but after 25 years of flashing her bits and pumping out the hits, Madonna's shock tactics are starting to feel a bit weary. And this is the problem with tonight's glitzy show.
This year, Madonna released her 11th studio album, Hard Candy, a fast and furious album of hip-hop-influenced pop. Its shock value lay in its glut of sexual references, and "Candy Shop", the track that kicks off the set tonight, is like a primer for the pervy pentagenarian. "Come into my store, I've got candy galore," she purrs, like a filthy Willy Wonka. "Don't pretend you're not hungry,/ I've seen it before." She then sings about lollipops, chocolate kisses and how her "sugar is raw" through several layers of electronic manipulation, and it's all a bit embarrassing.
Yet these antics are tame compared to Madonna's previous stage outings. Take 2005's disco-themed Confessions on a Dancefloor tour, which featured her strapped to a crucifix, wearing a crown of thorns. Further back, in 1990, she faked masturbation against the backdrop of a Catholic church, to the blasphemous strains of "Like a Virgin".
Religion seems off-limits for the Kabbalah-lover now, although she tackles politics tonight - most notably by putting John McCain next to Hitler and Robert Mugabe in a video montage. This feels predictable, especially when it's followed by a good-guy sequence in which Gandhi and John Lennon prop up Barack Obama.
Madonna really dazzles when she lays off the big subjects and attempts humour. Take the way in which she livens up "The Beat Goes On", a weak track from her recent album, by driving down the runway in a white convertible car and then pushing it back like an irksome mechanic. Later, she terrorises four mannequins representing different stages of her career, before putting on one of their wigs and crawling camply away. The crowd respond to this daftness by screaming their hearts out. Given most of them are dressed in pink cowboy hats and feather boas that's not surprising - it's the kind of fun they crave.
But when the humour dies away, Madonna can come across like a petulant teenager. This happens most when she plugs in her guitar, a prop that she has become increasingly fond of since 2000's cowboy-inspired album, Music. If she was a good guitarist, I'd salute her, but she obviously hasn't bothered to practise much in the past eight years. Her dull thrums ruin 2005's brilliant "Hung Up", and 1998's "Ray of Light" is even worse, ending with her sliding her plectrum awkwardly across the strings and dry-humping the speaker. You think of proper female guitar pioneers like Joan Jett and it's enough to make you weep.
And with every swear word she utters, it becomes clearer how forced is Madonna's air of rebellion. It's been three years since she said "motherfuckers" in front of an international TV audience for Live 8, and now her use of the word is boring rather than shocking.
Nevertheless, there are some wonderful moments tonight. "Like a Prayer", from 1989, is played as a mash-up with the Prodigy's rave classic "No Good", confirming its status as one of pop's greatest songs. Her recent number one, "Four Minutes", too, is impressive, although it is irritating that the main vocal riff - the sound of Madonna going "tick tock, tick tock, tick tock" - keeps popping up throughout the two-hour set.
Then again, this little tic says everything. On the one hand, it shows how Madonna's bullishness is starting to smother her legacy. On the other, it shows an artist who wants to push her present rather than her past. More than anything, it shows her commendable determination to fight against the clock.
Pick of the week
Echo and the Bunnymen
16 September, Royal Albert Hall, London SW7
Epic Eighties rockers join forces with the London Symphony Orchestra.
17 September, Manchester Academy
Hotly tipped indie band take their girl-group-flavoured music on tour.
21 September, Queen Elizabeth Hall, London SE1
A night of salsa and bhangra.