Pop quiz: What does Korean pop star Psy and English singer-songwriter Ed Sheeran's grandfather have in common?
Answer: They both had a hand in Sheeran's newest album, x, which has just become the fastest-selling British album of the year.
Afire Love, the third of seven singles from the album, is "about my grandfather", the 23-year-old Brit Award winner tells Life! in a telephone interview from Los Angeles, where he is on tour.
He says he started writing it a few weeks before his grandfather, who suffered from Alzheimer's disease, died late last year. He completed the lyrics after the funeral.
He sings through the pain: "And things were all good yesterday/ Then the devil took your breath away/And now we're left here in the pain/Black suit, black tie standin' in the rain/And now my family is one again."
It is not all doom and gloom in the album, though.
The single Sing, released in April, two months before the full album went public, was inspired by a night out.
More interesting is the music video that accompanies the song - that was inspired by the time he went out for a night of karaoke with the Gangnam Style star.
The pair first met when they both attended the Billboard Music Awards in Las Vegas last year and have been in contact since.
Psy does not appear in the video but it does feature a wild, puppet version of Sheeran partying at karaoke and strip clubs and eventually getting arrested by the police.
"Oh yeah, I got to keep the puppet," Sheeran, a man of few words, says when asked about the video.
The album title, pronounced as "multiply" and not the letter x, was inspired by how he "was trying to multiply the size of the songs and the size of the production".
He adds: "It was a three-year process and I just wrote as many songs as possible."
He wrote 120 songs but ended up choosing only 12 for the album, which was released on June 23. He says: "I just made sure I listened to them as many times as possible and deleted the s**t ones."
His decisions have proven to be right on the money. Last weekend, the album sold 182,000 copies, beating the previous bestseller, Coldplay's Ghost Stories, by 14,000.
British music critics have given it mixed reviews though.
The Telegraph gave it a perfect score, praising Sheeran's "effortless absorption of contemporary musical styles". The Observer gave it only two out of five stars, saying that the songs "don't actually find a new gear for the love song, just new turns of phrase".