Just an experiment. Nothing more. John is just a friend. Sherlock repeated this mantra in his head, but a nasty voice responded every time with such as: John is not just a friend, he's The Friend. Your only friend on who you can blindly rely. Don't ruin this by a silly experiment And: You were jealous yesterday, you know that bloody well. Don't let an experiment bet between you.
The feeling he had, while touching John, made the persistent thoughts even worse. He didn't want to stop the experiment, though. Not in the slightest.
Sherlock brought his lips even closer, his lips brushing behind John's ear and whispered: "It's quickening, you know." He took a step forward, his feet between John's. His body heat was almost tangible between them.