Bath Water

It was something she didn't really understand. For some reason, only royals could bestow magic by taking a bath. They get in a bath, masturbate, and then the water somehow ends up being filled with mana as a result. The water is then bottled and sold as mana potions. When drunk, the mana is absorbed by the body and the person can do magic until it runs out.

The more of the bath water someone drinks, the more mana they'd get. Not just that but the stronger the orgasm the royal has in the bath, or the more times they orgasm while in the bath, the more mana the bath water will get.

Of course, someone can get the most amount of mana by drinking the water while the royal is still in it. Because of all these factors, the princess is often joined by someone while in the bath.