Forgath lifted a hand, coating it in mana and frost as he created a shard of ice in his palm. "And that brings us to the first issue.  Most mana has a form or flavor to it, making it more useful for some spells than others.  Reaching into the frigid land around us makes it easy for me to work cold, but I could not use it to power my flames easily.  Converting the mana from one form to another is possible, though often costly; you end up spending mana to make the change, and need spellforms or enchantments specifically to do so.  Alternately, you can take it into yourself and change it into your own, but doing so is a risk; take in too much or without caution and the nature of you and your mana can be changed in turn.  That is why most of the exercises you do to expand your personal stores involve your own mana.