The funny thing about a good parody...

I think that I shall never see

A poem lovely as a tree;

In fact, until the billboards fall,

I'll never see a tree at all.

So wrote Ogden Nash, in a verse that caught my attention when I was a teenager, for at least three reasons.

First, I didn't know what a billboard was and had to look it up and find out it was the American word for an advertising hoarding, or what we would now call a prime poster site.

Second, I didn't realise till that moment that not all Americans were mad keen for Americanisation, and that some preferred trees to posters. Later I was to discover Americans who even preferred real cooking to McDonald's hamburgers. Third, it was obvious that Nash was parodying some well- known verse when he said " a poem lovely as a tree...", but it was a poem that I had never come across. Yet in a way that didn't really matter. It is one of the strange properties of good parody that you can deduce from it what the original was like even if you have never read it. My father, for instance, was in the habit of declaiming pieces of parody which I found funny even though I had no idea what the original was, such as the one that started:

It was Christmas Day in the mortuary,

The coldest day in the year,

When a corpse sat up and suddenly said,

It's bloody cold in here !

Then in came the mortuary-keeper,

His face all aflame with beer,

Took one look at him and said,

You can't do that there 'ere!

It wasn't till years later that I realised that this was a parody of all those tearful ballads with titles like, "It was Christmas Day in the Workhouse", but even at the age of eight or nine I knew that my father hadn't made it up, and that whoever he had got it from hadn't made it up, and that somewhere there was an Urtext. Similarly, when my father declaimed:

The boy stood on the burning deck,

His pockets full of bombs,

When one went off, the lot went off

And left him in his coms

I knew that he had left a lot unspoken there. First of all, I didn't know what coms were, and my father had to explain painstakingly that combinations were another name for long johns. Then, because nobody in our house had any long johns, he had to explain painstakingly the nature and purpose of long underwear. After which he had to explain that there was a very sad poem which began:

The boy stood on the burning deck

Whence all but he had fled

The flame that lit the battle's wreck

Shone round him o'er the dead.

Explaining parodies to me usually took longer than reciting them, which may be why he gave up explaining them after a while. For instance, he never explained to me the origin of another parody which he used to happily produce when I was off to school again:

There is a happy land, Far far away

Where they have ham and eggs, Three times a day.

Oh, how those boys do yell

When they hear the breakfast bell!

Oh how those eggs do smell

Three times a day!

I think it may be based on a hymn of the same opening line, but I never sang that hymn in my church-going days, so I am not likely to now. No matter. Now that my father is dead, and I am an orphan, I have belatedly started looking up the sources of his parodies, and have discovered that Mrs Hemans, who wrote "The Boy Stood on the Burning Deck" also wrote the poem that starts "The stately homes of England". I have also discovered from an old Oxford Dictionary of Quotations that it was Joyce Kilmer (1888- 1918) who wrote "/A poem lovely as a tree" though I have not the faintest idea who Joyce Kilmer was or what else she did in her short life.

In fact, when I sat down this morning I had no intention of saying anything about poetry and parody. It was billboards I wanted to talk about. Especially one billboard I saw last week. I happened to have to drive across England and back, which meant I was exposed to a wider selection of posters than usual, and I kept seeing one which began, in big letters, something like this:

"Alexander the Great had already conquered Europe by the age of 25..."

Perhaps we could compare notes on that one tomorrow, but I don't mind telling you now that that this is one advertising claim that worries me horribly.