Maybe because it has a bit of everything - adventure, humour, romance, suspense... It's perfectly understandable to children but the humour appeals to adults - so many films classed as 'family entertainment' are really just films for children, but this one really does work for all ages.

It also has all the appeal of a fairy tale but makes the genre funny, even dark. The villains, like Prince Humperdinck, are really satisfying characters, not cardboard cut-outs, and the old story of the handsome prince (or farm boy turned pirate, in this case) winning the heart of a fair princess is made unpredictable and, again, funny.

The action set pieces like Westley's fight with Inigo are superb, as good as you would find in an action movie, and the framing narrative draws the reader in (a boy who thinks he hates silly old stories is being read this story by his grandfather, and gradually falls under its spell).

It's also a good idea to read the original novel - just as much of a classic as the film.