Five things to know: At least 22 people – including children – are dead after an explosion at a concert in Manchester, England; a new study raises questions about the effectiveness of steroid injections in combating arthritis in knees; experts speak out about the sea lion and the little girl in Steveston, B.C.; treasure hunters dig up incredible relics from the River Thames; and a remarkable squadron of pilots raise awareness about aviators' role in the First World War.
