On 10 July 2010, Waters and Gilmour performed together at a charity event for the Hoping Foundation. The event, which raised money for Palestinian children, took place at Kiddington Hall in Oxfordshire, England, with an audience of approximately 200. In return for Waters's appearance at the event, Gilmour performed "Comfortably Numb" at Waters's performance of "The Wall" at the London O2 Arena on 12 May 2011, singing the choruses and playing the two guitar solos. Mason also joined, playing tambourine for "Outside the Wall" with Gilmour on mandolin.