Jokes, a jack-in-the-box, jelly and jumping beans make children laugh.

As do practical jokes, peekaboo, pantomine and poetry that makes no sense.

Why and how does this work? And why does it matter?

Writer and Professor of Children's Literature Michael Rosen, whose books - from We're Going on a Bear Hunt to Chocolate Cake - have made millions of children rock with laughter, gives us the tools for this greatest of gifts.