If you’ve ever played Magic Online, sooner or later you’re going to experience the frustration of clicking too rapidly through an X spell, like Bonfire of the Damned, and ending up casting it for X=0. Luckily, in the real world we’re a little more forgiving. If you cast an X spell without announcing the value for X, the Magic Tournament Rules prescribe a shortcut that assumes the value of X is whatever amount of mana is remaining floating in your mana pool. Tap a bunch of lands then say “Bonfire”? That counts. If you’ve got mana floating in your mana pool from another effect like Battle Hymn, that counts too. If Arcane Melee’s on the battlefield, factor in the cost reduction to whatever X is assumed to be. Magic is not a game of Gotcha, so no trying to be sneaky and claiming your opponent cast a spell for X=0 just because she didn’t verbally announce it!
The other, perhaps even more important reason for the shortcut is to avoid people trying to be tricky. You can’t tap for 8 mana and just say “Fireball you!” then when they cast Mana Leak decide that you only cast the Fireball for 4 and you have 3 mana free to pay for that Mana Leak. You’re assumed to have cast it for 7. If you hadn’t, then you committed a game rule violation by not announcing your floating mana when you passed priority (yes, that’s a thing you have to do!).
Today’s Tournament Tip written by Jen Wong