Celestia has needs that her girlfriend, Twilight Sparkle, doesn't particularly understand. She decides that the best way to explain herself is by hands-on demonstration. To help in her task, she employs her 'evil' half, Tyrant Sun. Surely nothing could go wrong with this plan...

Part of a pair. The other is over yonder.

Fair warning, this is a humanized Equestria story (but not an EQ Girls story). There's an OC involved too, along with a a bit of light S&M and an...unorthodox magical trick or two. And a quick word on human-ed pony physiology. Where a unicorn might have a horn, humanized unicorns have a gem, usually in the shape their cutie mark would be. They're also sensitive to touch, and tend to discharge magic at random when suitably excited. Wings still exist, but the Princesses can banish theirs at will, because they get the best of everything. And totally not because I kind of forgot about them. They are also sensitive.