Find out why The Very Hungry Caterpillar (pictured), 35 years old and still voracious, has sold 20 million copies worldwide. Whistledown Productions's delightful documentary about Eric Carle's classic picture book is on BBC Radio 4 at 3.30pm tomorrow. What is the enduring appeal of those 14 pages and 224 words? The Caterpillar aficionados talking to John Hegley include a teacher of children with severe emotional and behavioural difficulties who finds it hits the spot for young children "hungry for attention but not hungry for learning". Carle himself believes the appeal is "the holes in the pages". Even George W Bush loves it - it's his first choice to read aloud when he visits schools - but we are left to ponder on the reasons.