Bubbles
The sight that met John's eyes as he entered the bedroom was nothing short of adorable. Sherlock was lying in bed, on his back, blowing bubbles at the ceiling.
"Bubbles, John," he explained, "are all unique. The structure of each soap film is different."
"And yet always the same shape."
"A simple matter of mathematics," Sherlock continued. "A sphere uses the minimum surface area to enclose a fixed volume." Here he paused to catch two bubbles on the wand. "Notice how the shape changes when the bubbles join together. It minimises the energy required."
"Is that so?" John said, smiling as he slid into the bed. He nestled his head against Sherlock's neck, dropping a light kiss on the other man's shoulder, and watched the bubbles float slowly down around them.
"Besides," Sherlock mused, "they are rather beautiful."