It was just a silly game, one long and lazy Sunday afternoon when we had nothing to do but fuck - a single, short line drawn in felt-tip on your shoulder blade, no bigger than half a matchstick, each time you came, every fifth criss-crossing the previous four, like a prisoner counting time.

'Nine...ten...eleven...'

But when we showered after, my soapy palms tracing your geometry, you asked me to leave the count intact, and I pressed my lips to the lines as we lay together again in the dark, and drifted into sleep.

And then you came home from work the next day with a bag, a Cheshire Cat grin, and plastic wrapped across your shoulder - scrubbed of the lines, but glowing as if sunburnt.

'What did you do?' I asked, examining the redness of the skin.

'I got them tattooed,' you answered, music in your voice as you pulled a strange black tube from the bag, 'with UV ink'. You flicked a switch on the tube and held it to your neck, and I saw the lines flare to life, reappearing in bright blue. 'It only lasts a year, and you can't see it without the blacklight.'

I was so overcome with delight, at your absurdity, at the romantic ridiculousness of what you'd done, that I immediately dropped to my knees, tugged your jeans to mid-thigh, and licked you from behind until you ground your shivering cunt against my nose. I drew another pair of lines, this time against your hip, and again the next day you visited the tattooist on your way home.

After a few weeks, I bought a bigger blacklight, screwed it to the wall above the bed, so I could watch the lines dance and ripple under my hands, my mouth, while we added to their number. They grew everywhere, sometimes in ones and twos, other times in great scattered clusters, sprawling across limbs, making a wireframe of your stomach, a tangled web wrapped around your thighs, your arse. I counted and kissed them by way of foreplay, taking my sweet, serene time as I tried to recall which encounter each came from, what act of quiet passion or furious lust created them, but inevitably lost my way, and myself in you, again and again.

Eventually the oldest lines started to fade, the glow subsiding a little more each passing week, the ink subsumed into your flesh, the evidence of your pleasure returning from whence it so spectacularly came. But we added their replacements, day after day, night after night, each of us carrying a pen in our pocket wherever we went, just in case the tally grew. And now, whenever I undress you, in the darkness of our bedroom's blackout blinds, we watch the whole of the heavens, constellation by blessed constellation, appear in my arms.