Arcane Catalyst

Whenever a spell you control with {x} in its mana
cost exactly once is put on the stack and the value
of X in the spell's rules text is set to zero, you
may pay {x}. If you do, you may replace the value
of X in the spell's rules text with the value of X
you chose when paying this cost.
"Nothing's truly impossible, it's just a matter of
timing and know-how." - Archmagus Diurnesis