When there's a low tide on the Thames, you get a kind of muddy beach on the riverside and brave people build sandcastles as if they are on a real holiday.
Sometimes on the South Bank's bank, they have impromptu mud-beach parties, complete with DJ's, blaring sound systems and that stumbling, young-at-heart middle-aged misfit all genuine raves must have.
Boris Johnson, 45, appears to have had enough of this haphazard arrangement and wants something more official, pointing to the success of city beach schemes in Berlin, New York and Paris.
"I'm envious of them," said the London Mayor. "It's certainly something we're looking at, but it's expensive."
You can just see the next step. "Expensive" means they'll charge for access to the beach, means it'll have ad hoardings and CCTV cameras and a load of middle-aged misfits in uniforms, truncheons at the ready, to keep the peace. Suddenly, mud, glorious, mud has some appeal.