Brain in a Jar — ruling 1

When resolving the first ability of Brain in a Jar, the newly-placed charge counter will be counted when determining what spells you may cast. No player may take any action between you placing the counter and choosing which spell to cast.

Brain in a Jar — ruling 2

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.

Brain in a Jar — ruling 3

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.

Brain in a Jar — ruling 4

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

Brain in a Jar : règles, fonctionnement et interactions (MTG)

Brain in a Jar is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

, : Put a charge counter on this artifact, then you may cast an instant or sorcery spell with mana value equal to the number of charge counters on this artifact from your hand without paying its mana cost. , , Remove X charge counters from this artifact: Scry X.

How it works : Brain in a Jar

Original English text — Wizards of the Coast / Scryfall

#1April 8, 2016· Wizards of the Coast
When resolving the first ability of Brain in a Jar, the newly-placed charge counter will be counted when determining what spells you may cast. No player may take any action between you placing the counter and choosing which spell to cast.
No other card is named in this ruling.
#2April 8, 2016· Wizards of the Coast
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.
No other card is named in this ruling.
#3April 8, 2016· Wizards of the Coast
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.
No other card is named in this ruling.
#4April 8, 2016· Wizards of the Coast
If the card has in its mana cost, you must choose 0 as the value of X.
No other card is named in this ruling.