It’s easy to assume these suds are left over from the scrubbing down of a pavement but they are, in fact, the byproduct of a unique Soho experiment in technologically enabled policing. During the 60s and subsequent 70s the police of Soho were notoriously and irredeemably corrupt, so much so that even ‘the authorities’ decided something should be done. They wanted a way to prove which officers had been where at which time and turned for help to the boffins of the GPO Foam Unit - who soon developed the Soho Soap Solution. The SSS emits streams of tiny bubbles from thousands of locations across Soho, each stream bearing the time and a unique identifier ‘stamped’ into it via the shape of the emitter. So, in the event of a corruption enquiry, individual officers can be dusted for bubbles and their alibis checked.

In much of London the system has been superseded by various ‘smart water’ schemes - aimed at criminals rather than the police - but only Soho’s has the jaunty air of a children’s party.