There are two Celestias. They're both princesses of the sun. They both care dearly for their subjects. They also have an unhealthy obsession for cake. Mentally they're the same.

But physically, they're different, and out of curiosity, they're going to explore each other's bodies to learn the ins and outs of each others bodies. But mostly ins.

Birthday present for the Wet and Slippery Sponge

Thanks to TheFIMguru for his help editing