There are times, particularly in a Monday morning maths lesson, or while shivering on an icy netball court, when there doesn't seem very much to be happy about. Other members of staff don't do much to dispute the theory. The head is far from ecstatic - often all that can be seen of him is a thin pall of smoke denoting his presence in the lavatory.
Determined to cultivate a more optimistic attitude, I stride into my classroom of eight-year-olds having planned a lesson called "Reasons to be Cheerful". My breezy smile fades as I wipe a table on which someone has voluminously sneezed. But this is only a minor interruption. I explain, brightly, that there are many reasons to feel cheerful and happy, and we are going to try to think of some of the very best. Silence.
After a few desperate leading questions, I become aware of an atmosphere of gloom and dejection. I make further efforts.
"Think how lucky we are!" I sing gaily. "Haven't you ever seen anything that makes you realise you're lucky?" This strikes a chord.
"Y'aren't in a wheelchair, Miss."
"Y'aven't got yer legs off."
"An' yer arms missing."
There is no stopping the flow of grisly enjoyment.
"Y'aven't gorra disease."
These are all rather negative reasons for cheerfulness. What about the things we do have? This brings forth a fresh torrent.
"Got two arms."
"Got yer legs an' yer can walk. I knew a fella once . . ."
"Got yer eyes to see with."
I agree that I am fortunate in all these respects. But what other things do we have? Immediate regret. Boasts of Tamagotchis and mountain bikes fight for attention. In one corner there is even a row about bathrooms.
"Well, our bathroom's got two toilets, a big un an' a little un' that me mum sits on."
Spotting a vital teaching point, I boom over the din. "What do we call that thing some people have in their bathrooms? It looks like a small toilet, but it's not a toilet."
"It's fer washin' yer bum in," says the owner helpfully.
"It begins with a 'b-u-h'," I go on loudly.
"Bum-washer?" suggests a young hopeful.
Rejecting the bum-washer as a major reason for feeling cheerful, I press on. Eventually, other pleasures begin to surface. One boy has a window box of his own. He likes digging in it. I suddenly have an urge to go and dig myself. Someone else has touched a newly born kitten.
Gradually the lesson settles into a wallow of small and lovely delights. Someone likes the smell of peapods. Someone else likes the noise when you pop them.
After a while there is a rhythm of labouring pencils. Approaching an adenoidal child whose book makes the other children's look like Bede's Illuminated Bible, I read: "I like it when my little sister tickles my back."
I drive home, cheerfully.
There's a Snickers waiting in the fridge for Coronation Street, and there are three letters on the mat. Later, I have a bubble bath, and read in it till my fingers go pruny at the ends. It's a cheerful evening, too.
Jane Bower lives in Cambridge