But I was not laughing. Because as the words 'surely someone's seen' had left my mouth, so had something else. I had watched in horror as a small globule of spit had left my mouth and arced quickly and beautifully through the air. It was larger than a normal globule. This was no speck. This was the size of a basketball, if you were the size of an ant.

There were too many S words!, I think, in a panic. This was not my fault!

For a second, I think my spitball has simply disappeared into the ether, like a comet entering this man's atmosphere.

It burnt up! I think, with relief. It burnt up!

But no. Its graceful, looping arc had continued until it lolloped squarely on to the man's forehead, where it now quietly sat, all pleased with itself.

I dare to wonder if I may have got away with it. But I had seen a nanosecond of horror in the man's eyes. A micro-widening of his eyes and a moment of startled confusion.

He knows!

And now, as I squint ever so slightly, I can see it. Right between the man's eyebrows. Can he see it? Maybe he didn't see it coming! Maybe he thinks he's just sprouted an instant pimple! Or maybe he saw everything! And felt everything! Oh, God - what could that have felt like?

He snaps out of whatever he'd snapped into, and starts a new subject. I can tell he wants to get a hankie out and wipe his brow, but he doesn't. He says, 'Sometimes, when you come to these things ...' and I zone out, because I'm concentrating on where my spit has gone. I notice with relief that it's not dribbling. I am not a St Bernard and produce only an average amount of saliva for an adult human male, and so it's far too small to dribble, and what's more, the man has provided excellent surface tension. But it's still there. It's definitely still there. Just foam and a bubble, probably, but it glistens and twinkles and surely that's enough?

Just act normal, I am thinking. Even though you have just spat on a man's head.