The Law of Conservation of Energy states that: Energy, within a closed system, will remain the same. It cannot be created, nor destroyed, only change from one form to another.
In my magic system: Mana is quite literally a form of energy that is able to be directly controlled by a creature(say, a human) and is used to power magic. Mana exists across the entire universe, but not everyone can naturally control or even access it.
Initial Mana is where someone is born with a concentration of Mana inside their bodies. (roughly 50% of the human population have Initial Mana). Up until now, Init Mana was a big deal, because it never depleted (except in rare cases, which would weaken or potentially kill its host). Those without Init Mana had to carry a supply of Mana around, which could be depleted quite quickly depending on the context.
Init Mana doesn't abide by this law because it is sustained even though it can have tremendous energy drawn from it, assuming the draw is less than their internal Mana capacity. (like a 5v fan running infinitely as long as it didn't somehow draw 6v)
I need to find a way to explain this within that law, without breaking what makes Init Mana so great.
Also, Init Mana is written to be slightly different than regular Mana, so I could make up rules to explain this without risking breaking things.
My magical system is much more detailed than this, so if you need more information, please don't hesitate to ask!