Powerbalance

If the revealed card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If an opponent casts a spell with {X} in its mana cost, use the value of X that was chosen when it was cast to determine its mana value.

In some unusual cases, the card you reveal from the top of your library may not have the same mana value as the spell your opponent cast, but you can still cast that card because the resulting spell does have the same mana value. For example, if an opponent casts a spell with mana value 4 and you reveal Frogmyr Enforcer (a prototype card with mana cost {7} and prototype cost {3}{R}), you can cast Frogmyr Enforcer as a prototyped spell, as that spell will have a mana value of 4.