If you can tell a lie by the age of two, apparently, you're a smart child who will go far. How smart you have to be to get away with it, though, is another matter. In small children, it's easy to spot when they're lying, researchers say. No, it's not actually true that their noses lengthen alarmingly. Apparently, a small child who is telling the truth looks to the left, while a child who is fibbing will look to the right. Sorted.
Well, no, because as they get older, of course, they get better at it, and lies become more credible. Older students can lie beautifully. Jamie came back from lunch late and I suspected he'd been down the pub, partly because the rest of the class clyped. "Have you been drinking?" Accusingly.
"No - course not. I had to go to my gran's," and there was a long complicated explanation.
I let the matter drop. Then, as I explained what he'd missed, I added casually: "So was the pub busy?" Jamie shook his head. "No, pretty quiet really."
At two or 20, fibbing is often a rebellion - a way of making your stand against authority, of refusing to take a telling off. And it's not just porky pies. We know our young people are programmed to rebel, yet we still seem to set up systems that invite the "oh yeah?" response.
I was always taken aback by how many of the girls in my classes were gasping for a fag come break time. They'd rummage in their bags for the essentials - mobile, lippy, purse, fags. More and more young girls, it seems, are taking up the habit despite anti-smoking campaigns and stark warnings on the packets.
Blame celebrities? Young people are still bombarded with images that say smoking is cool. But worryingly, it has been suggested that the campaigns and stark warnings themselves have encouraged these girls to light up. It's the "tell me not to do it and I'll do it anyway" syndrome that we thought kids grew out of after the terrible twos.
I used to show younger classes a video on how a popular soft drink was manufactured. It included warnings from a dentist and a fairly graphic account of what sugary drinks did to your teeth and mouth. I stopped showing it when I realised the effect of the video was to send them scuttling off at break to buy a bottle.
Why can't being "cool" involve something that's really good for you, like eating apples or having two early nights a week? Maybe it's our fault. We get the message wrong and we provoke the very behaviours we seek to protect our young people from.
And Jamie? His work wasn't up to his usual high standard that afternoon. I went over it with him. "You don't like it?" he asked, downcast.
"I'm . disappointed," I said.
Next to him, Julie groaned in sympathy. "Oh that's so cruel," she said. "It's like when your mum says she's disappointed in you - it makes you feel so bad."
We need to get the message right; to find a way to make our young people feel bad about things that can harm them, and to feel good about things that will help them live long and happy lives.
Carol Gow is a former FE lecturer in media.