John stumbled down the stairs, blinking blearily. Sherlock was sitting on the floor of the living room, a bubble wand in one hand and a bottle of soapy solution in the other. A series of panes of glass were resting on the sofa. As he watched, Sherlock dipped the wand, pursed those ridiculous lips, and let out a controlled breath, causing a cascade of bubbles to float across the room and pop against the glass, leaving a series of soapy rings on the glass. Watching him pucker his lips like that should have been arousing, but there was something so charming and child-like about it that John couldn't see it as anything else.
"Isn't it fascinating, John?" Sherlock leaned over, a look of barely-disguised glee spreading across his face. "Despite looking nearly identical, every bubble is entirely unique, as are the prints they leave. They're almost as different as fingerprints!" John giggled at the excitement on Sherlock's face, and was rewarded by a bombardment of bubbles in the face, causing another fit of laughter.
If you'd asked John before that morning what the cutest thing in the world was, he'd have said something insipid and uninspired do with babies, kittens or similar rot. However, if you'd asked him after that morning, the answer would have been unequivocally Sherlock Holmes, blowing bubbles.