He greatly enjoyed this experience as it was a positive message, whereas most concept art he encountered at the time was anti-everything. Lennon was also intrigued by Ono's Hammer a Nail where viewers were invited to hammer a nail into a wooden board painted white. Although the exhibition had not yet opened, Lennon wanted to hammer a nail into the clean board, but Ono stopped him. Dunbar asked her, "Don't you know who this is? He's a millionaire! He might buy it." Ono feigned not knowing of the Beatles (even as she had gone to see Paul McCartney asking for a Beatle song score), but relented on the condition that Lennon pay her five shillings, to which Lennon replied, "I'll give you an imaginary five shillings and hammer an imaginary nail in." In a 2002 interview, Ono said, "I was very attracted to him. It was a really strange situation." Ono started writing to Lennon, sending him her conceptual artworks, and soon the two began corresponding. In September 1967, Lennon sponsored Ono's solo Half-A-Wind Show, at Lisson Gallery in London. When Lennon's wife Cynthia asked for an explanation of why Ono was telephoning them at home, he told her that Ono was only trying to obtain money for her "avant-garde bullshit". In early 1968, while the Beatles were making their visit to India, Lennon wrote the song "Julia" and included a reference to Ono: "Ocean child calls me", referring to the translation of Yoko's Japanese spelling. In May 1968, while his wife was on holiday in Greece, Lennon invited Ono to visit. They spent the night recording a selection of avant-garde tape loops, after which, he said, they "made love at dawn". The recordings made by the two during this session ultimately became their first collaborative album, the musique concrete work Unfinished Music No. 1: Two Virgins. When Lennon's wife returned home, she found Ono wearing her bathrobe and drinking tea with Lennon, who simply said, "Oh, hi." On September 24 and 25, 1968, Lennon wrote and recorded "Happiness Is a Warm Gun", which contains sexual references to Ono. Ono became pregnant, but she suffered the miscarriage of a male child on November 21, 1968, a few weeks after Lennon's divorce from Cynthia was granted. On December 12, 1968, Lennon and Ono participated in the BBC documentary about The Rolling Stones, The Rolling Stones Rock And Roll Circus, along with