There's a myriad of different kisses.

If it isn't for the fact that the soft sciences are an anathema, he'd think about trying to classify kisses, herd them into neat genera and species, tabulate them, control them. A really dedicated scientist would produce the osculatory equivalent of the Periodic Table, everything neatly labelled and categorised, each kind of kiss carrying its academic footnote to cite source data and research methodologies.

If he could spare the brain cells, he would be able to see it inside his head, this classification of kisses.

The lightweight kiss first; that first tentative pressing together of juvenile lips that had you wondering what all the fuss was about. And once he has that one firmly in its place, he'll have to work his way up through all the varieties of kisses that you get after you discover (through experimentation, of course, although the data sets were a little sparse in his teenage years) that if you just open your mouth and, you know, kind of move things around a bit, your tongue suddenly has a lot more positive uses than just allowing you to articulate your best insults clearly and to swallow things without choking.

Soft kisses and hard kisses. Kisses to the cheek from mothers and maiden aunts, and kisses to the cheek from defaulting lovers. Kisses that signal love and kisses that only touch air. Kisses that are promises, and kisses that are lies. Kisses so light and feathery that you can barely feel them and kisses that are so potent that their weight crushes you down. Kisses that are wet and slobbery with people who don't know exactly how to hold their lips to get the best and sexiest effect, and wet and sexy kisses with people who do. Kisses that turn the blood to molten lava and kisses that cool you as you come down. Kisses that inflame and kisses that soothe. Kisses that mean only good fellowship and casual affection, and kisses that are desire incarnate.

And now he has to add slow kisses to the classification. Kisses so leisurely and intense, so measured and considered, that the world comes to a stop while a hot tongue moves over his lips and explores each and every tooth, while teeth pull gently at his bottom lip, biting it softly until his lip is swollen and hot and heavy. He has to lick his lip to cool it and meets Sheppard's tongue with his, and only then does Sheppard's mouth close over his, and something jumps in his chest, hot and hungry and clamouring.

Slow kisses. Deliberate kisses. Unhurried kisses.

"You're over-analysing this," says Sheppard, laughing. His eyes are wide and hot, and his mouth is there, waiting to be experimented upon, studied and evaluated, compared against existing data for compliance and deviation.

"I think I need more empirical data," gasps Rodney, his lips against Sheppard's throat. He feels the vibration as Sheppard chuckles. "Repeat the research."

"As long as I get a mention when you win the Nobel," says Sheppard.

And he starts again; kissing Rodney, hands mapping Rodney out and smoothing down Rodney's sides until Rodney's trembling and the whole of unified field theory could unfold against the inside of his closed eyelids in letters and numerals of burnished gold, and Rodney wouldn't notice.