Brain in a Jar

If the card has X generic mana in its mana cost, you must choose 0 as the value of X.

If Brain in a Jar leaves the battlefield before its first ability resolves, use the number of counters on it at the moment it left to determine what spell you may cast. That number won’t change because you can’t put a new counter on Brain in a Jar.

If you cast a card “without paying its mana cost,” you can’t choose to cast it for any alternative costs, such as awaken costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, such as that of Lightning Axe, you must pay those to cast the card.