Go into any book store or library and you will see books divided up into various categories, generally on subject matter.
You’ve got chick lit, thrillers, romance, biographies and, of course, an entire section usually bypassed by adults - children’s books.
But some stories are just too good to be restricted by conventional boundaries.
Harry Potter was a great example of this and now War Horse is proving to be a hit with all ages too.
Written by former children’s laureate Michael Morpurgo nearly 30 years ago, it has now hit the big screen.
And what an epic it is.
Morpurgo took the massive subject of the First World War and had the brainwave of telling it from the point of view of one of the horses used by the British Army during this deadliest of conflicts.
A million horses were killed during the four years that war raged - and that was just on the British side. The total number of human casualties on all sides was a staggering 35 million.
Judging by those figures, this might seem like unsuitable material for a young audience.
But both the book and the film deal with it sensitively, while never glossing over the appalling nature of this war.
Director Steven Spielberg proves his cinematic genius once again by creating a masterpiece which deserves its five Bafta nominations.
It’s refreshing to learn that he avoided CGI trickery and instead used eight real horses to play the starring role of Joey. Whoever trained them deserves an award of their own.
With a 12A rating and at two hours and 20 minutes long, I was slightly concerned that my eight-year-old might find it too heavy going.
I needn’t have worried though, he enjoyed it just as much as his 14-year-old brother and 12-year-old sister.
Maybe it was something to do with that affinity children often have with animals, but they each sat spellbound as the story of Joey and his young master Albert Narracott (Jeremy Irvine) unfolded.
Irvine is a new name to me but he was superb as the teenage lad who forms a lasting bond with the part-thoroughbred which his drunken father Ted (Peter Mullan) bought as a work horse.
Although Albert manages to get Joey to work a plough, when war is declared in the summer of 1914, Ted’s money worries lead him to sell the horse to the army.
Heartbroken Albert promises Joey that he will find him again and, as soon as he is old enough, he joins up and is sent to the trenches in France.
Joey becomes the chosen mount of Capt Nicholls (Tom Hiddleston) and is trained for combat.
The first time we see the cavalry charge into a German camp it brings home the ridiculousness of the British approach to this war.
With their swords outstretched before them, the mounted soldiers are no match for the heavy artillery which is turned upon them.
Horses were eventually taken off front line duty and were used instead to drag guns and ambulance carts instead.
This is the fate which eventually befalls Joey and his horse companion Topthorn - a black stallion which becomes his best friend - when they end up being taken by the German army.
The scene when Joey escapes and runs scared through No Man’s Land, terrified by the shells exploding all around him, is one of those movie moments which live on in the memory long after the credits have rolled.
The film differs a bit from the book, but it remains true to the spirt of this thought-provoking, but ultimately rewarding, story.
by Helen Johnston