How to explain infinity to a 4-year-old

‘What’s the number before infinity?’ asks Claudia, aged 4. We challenge Johnny Ball, legendary British TV presenter, to explain. And in celebration of the voice of Sesame Street’s Count von Count, Jerry Nelson, who’s died aged 78, there’s another chance to hear our 2009 interview with the Count, in which he revealed his favourite number: 34,969. Presented by Ruth Alexander, this programme was first broadcast on the BBC World Service.