Los Angeles: Singer Katy Perry sang for Isla Fisher at her 40th birthday party in February, the actress has revealed after months. Fisher, who turned 40 on February 3, has said that Perry was asked by her husband Sacha Baron Cohen to serenade her at a family party, reports femalefirst.co.uk. “I celebrated my 40th birthday with my friends, family - and Katy Perry. My husband (Sacha Baron Cohen) asked her to sing for me and she kindly obliged. I had to pretend that I didn't know the words to every single song so she wouldn't realise how big a fan I am,” Fisher told YOU magazine. Fisher, who has children Olive, eight, Elula, five and 14-month-old son Montgomery with husband Baron, said her biggest aim in life is to be "considered cool" by them. “My biggest aim is to be considered cool by my kids. I'd also like to write more children's books and, on the acting side, work with Coen brothers, David Fincher, Spike Jonze and Adam McKay - preferably, not as their assistant,” she said.