A giant pair of girl's knees.
The reason? That's what he was seeing on his visit to London at the time. The flesh on display between the top of a pair of boots and the hem of the mini skirt.
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The sculpture was never built but, displayed in the Barbican as part of the current Pop Art Design exhibition, there is a model of it, entitled London Knees, made in 1966. It is one of the few life-sized artworks on display in the exhibition and is sandwiched between a giant thumb and a giant foot.
Pop Art Design is an exhibition that's all about scale and youth and colour. It is an attempt to look at how art in the 50s, 60s and 70s fed into the design of the time and vice-versa and it does a perfectly good job at doing so.
But what you take away is a kind of retina dazzle, after being assaulted by graphic boldness, primary colours and deliberately simplified imagery.
Pop Art Design is all about consumer culture, advertising, celebrity and sex.
You can see those elements in the Italian Studio 65's Bocca, a polyurethane foam sofa shaped like a pair of giant red lips, in Peter Blake's 1962 painting of the Beatles that apes an album cover and in Steinberg's curvaceous illustration of a naked woman drawn onto the curvaceous planes of an Eames chair.
Actually, you can see them in almost everything on display here.
This is very much a vision of the high 60s on the upbeat.
That's partly about curatorial choice. Responses to the Vietnam war and the assassination of JFK are mostly passed over here.
There are Warhol cherubs and Coke bottles on display but none of his images of Jackie Kennedy based on photographs of her on that day in Dallas.
It's also partly a reflection of the optimism of the time, because, despite everything, the idea of change - however socially difficult - was energising.
The result is often cartoonish. It's clear that both art and design in the time were drawing on similar influences. Gigantism, bloopy, gloopy lines, Americana.
It's a comic-book vision of the world (and in fact there are some 60s comic books in the exhibition, including a Jim Steranko episode of Captain America that is as iconic as anything else on show).
There's a very 60s cheek and satire on display, too. Religious icons turn into pin-up girls, a curvy sofa is covered in the stars and stripes (another Studio 65 creation), everyday objects (coke bottles, light switches, ice creams) are elevated - and expanded - to artistic subject matter.
Pop art's veneer of irony has cast a long and increasingly corrosive shadow but here it still feels fresh and vivid (even when you can see the wear and tear).
It's striking how well the design stands up against the art.
The sexiest thing in the exhibition is not Evelyne Axell's incredibly rude portrait of a girl licking an ice cream or Tom Wesselmann's rouged lips with cigarette but Gaetano Pesce's La Mamma Donna, a blobby chair and footrest in shrieking red that looks as if it came straight off the set of Barbarella.
It's an evocation of sensuality that doesn't come flashing neon lights or dated objectifications of the female form.
The same cannot not be said for its exhibition neighbour, Allen Jones's Chair, in which a half-naked female form raises her legs to give you somewhere to sit down. What may once have looked provocative now looks shabby and offensive.
Amid all the glam and glitter and the odd giant Anglepoise lamp (courtesy of the father of British pop art, Richard Hamilton) there's a also a melancholy to this exhibition, it should be said.
Because in the end it speaks of a time in art and design when optimism, energy and creativity were high.
It also speaks of a time - and these things are not unconnected - when the economy in the West was flourishing.
This used to be our future. In the 21st century this exhibition represents the bright plastic hopeful past we've been passed out of. It is shining in our rear-view mirror, receding from view further and further, until...
Pop Art Design continues at the Barbican Art Gallery, London until February 9