Mental magic can be played with several different rules.
- Any card can be played as any card that excatly the same mana cost. So Rampant Growth can be a Tarmogoyf of Deathcap Cultivator depending what you need on that moment.
- Once a card has been named you can’t name it again.
Reverse mental magic
- When you play a card your opponent decides which card you just played that has exactly same mana cost.
Alternative rules to start the game easier.
- You can use pretty much any pile of cards or even a deck for this
- Each card can be played face down as a utopia land that produces any color mana but is not considered a basic land.
- Once a turn you can discard a card to get any basic land.
- No searching cards from decks.
Read more:
https://mtg.gamepedia.com/Mental_Magic
MTG blog 