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A: Matt Tabak, Magic rules manager:
Greetings, citizen. Many spells in Magic have abilities that let you pay what's known as an alternative cost. That is, instead of paying a spell's mana cost, you pay some other cost, often with a bonus for doing so. Bestow is one such ability. Any effect that says you can cast a spell without paying its mana cost is also an alternative cost. Unfortunately, you can use a maximum of one alternative cost for each spell you cast. So you can cast a spell for its bestow cost, or you can cast a spell without paying its mana cost, but you can't do both.