Casting spells with "X" in their mana costs without paying mana cost

[AMAZINSPIDERFAN]

I have a mono-red burn deck. I have Possibility Storm in it and 3 or 4 Volcanic Geysers. I was just wondering, should it so happen, what if the card I would get to cast without paying mana cost has an "X" mana cost? Would I still have to pay X? Or would the spell just fizzle? Surely I can't just say that X is whatever I want it to be lol.

[MuggyWuggy]

Rizzy dizzy
(RESOLVED)

[particle]

i feel like many rules would answer themselves if people read the gatherer

[MuggyWuggy]

Unfortunately the Internet encourages "ask google" research standards