During this cruelly hot and sunny exam season, teachers and parents are crucially aware of the importance of motivation. When the balmy outside air is full of the smell of barbecues and the sound of chinking glasses (or the smell of dope and the sound of friends joshing each other on the corner), it is hard to stay in a stuffy house and slave over maths or history.
The grown-ups know that there is not much more they can do now - apart from producing food regularly, offering a shoulder to cry on, making sure that their bleary-eyed candidates get to their exams (teachers, it seems, are taking on this responsibility too).
But mostly it is down to the young themselves. And we wrestle once more with the perplexing question of why some young people try so hard while others hardly make any effort at all. Of course, the most unlucky exam candidates struggle with challenging family circumstances which offer them precious little support - although even among the bleakest high-rises we can see astonishing examples of tenacity and commitment.
Then there are psychological factors, such as wanting to outperform an irritating sibling or to please a demanding father. Mrs Thatcher's famously high motivation is thought to have come from her wish to impress her redoubtable father Alderman Roberts. But I am sure the iron truly entered her soul when she arrived in glory at Oxford University, and found that "snobby Roberts", the star of Kesteven and Grantham girls' high school, was ignored and patronised by braying public-school boys as just another "northern chemist" with a peculiar accent. "I'll show 'em," she must have whispered to her pillow at night - and she did.
Such psychological motivation can be pretty powerful, but not for everyone.
One of the most fascinating facts about encouraging people to do well is that everyone has different buttons to be pushed - and gifted teachers tend to find them.
But teachers are not the only ones who can make a difference - as was very evident last week when I presented three Toshiba laptops to the winning Mentors of the Year at London's Guildhall. The national mentoring pilot project, now in its fourth year, involves more than 900 undergraduates at 19 universities, who are paired up with local schools to befriend and encourage 3,500 pupils at risk of faltering in their studies. What such teenagers need is someone focused entirely on them to help them manage their revision and get their coursework done, to discuss problems and offer a sympathetic ear. These big-hearted and energetic students fill the bill, while themselves gaining a great deal from the experience.
We know from the Office for Standards in Education that too many pupils are still giving up between the ages of 14 and 16. Hormones are rampant, peer groups highly influential, and academic work often seems dull and irrelevant. Parents and teachers find themselves mouthing platitudes which are contemptuously dismissed by their rebellious charges. But the great thing about these zesty mentors is that they too are young - still doing exams, in most cases. They can empathise with their mentees' confusion and act as convincing examples of what can be achieved with a bit of effort.
Charles Clarke reportedly wants more proof that mentoring makes a real difference before he agrees to fund it nationwide. This sounds like an excuse to avoid committing the cash. Because the evidence is under his nose - if he wants to look for it.