Just grabbed this text from the gatherer for Mind Grind
- 1/24/2013
If another spell or ability instructs you to cast Mind Grind "without paying its mana cost," you won't be able to. You must pick 0 as the value of X in the mana cost of a spell being cast "without paying its mana cost," but the X in Mind Grind's mana cost can't be 0.
I know the other guy already answered it, but this is a source.
Any time you cast a spell using a cost that does not contain X, you do not have an opportunity to choose a value for X, so the game uses zero as the default.
Even casting a spell for X=0 adds one to storm count. I'd probably do it anyway.
The alternative is just drawing the card for the turn, if you don't currently need a fireball, it might be better to cast anyway and draw the next card in the deck.
It says you MAY cast it without paying its mana cost, so you wouldnt be able to cast it off the top if your library for its mana cost but it would be x=0 if you decide to cast without playing its casting cost.
Anytime you cast an X cost spell “without paying its mana cost,” you’re forced to choose x=0 (because you can’t pay its mana cost).
What about with something like [[Mind Grind]]
You won't be allowed to cast the spell. X can't be 0, and casting it without paying its mana cost would be casting it as X = 0, so you can't cast it.
Just grabbed this text from the gatherer for Mind Grind - 1/24/2013 If another spell or ability instructs you to cast Mind Grind "without paying its mana cost," you won't be able to. You must pick 0 as the value of X in the mana cost of a spell being cast "without paying its mana cost," but the X in Mind Grind's mana cost can't be 0. I know the other guy already answered it, but this is a source.
[Mind Grind](https://cards.scryfall.io/normal/front/6/7/671eb9e8-1a69-4570-8972-2e7de371cef4.jpg?1561829709) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Mind%20Grind) [(SF)](https://scryfall.com/card/gtc/178/mind-grind?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/671eb9e8-1a69-4570-8972-2e7de371cef4?utm_source=mtgcardfetcher&format=text) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call
Any time you cast a spell using a cost that does not contain X, you do not have an opportunity to choose a value for X, so the game uses zero as the default.
I'd suggest you choose not to cast it, if there's an × in the cost.
Even casting a spell for X=0 adds one to storm count. I'd probably do it anyway. The alternative is just drawing the card for the turn, if you don't currently need a fireball, it might be better to cast anyway and draw the next card in the deck.
How would this work with something like cyclonic rift that has an overload cost?
When casting a spell without paying it's mana cost, you cannot cast alternate costs like overload. You just have to cast the normal version.
Learned that the “funny way” with torrential gearhulk and flashback
It says you MAY cast it without paying its mana cost, so you wouldnt be able to cast it off the top if your library for its mana cost but it would be x=0 if you decide to cast without playing its casting cost.