Creature â Homunculus
: Add . Spend this mana only to cast an instant or sorcery spell.
At the beginning of your upkeep, if there are three or more instant and/or sorcery cards in your graveyard, transform Curious Homunculus.
Mana from Curious Homunculusâ€™s first ability can be spent to cast a spell that has generic mana in its additional cost, such as Borrowed Malevolence with both modes chosen. It canâ€™t be spent to pay costs during a spellâ€™s resolution, such as the cost Convolute prompts a player to pay.
If you donâ€™t have enough instant and/or sorcery cards in your graveyard as your upkeep begins, Curious Homunculusâ€™s last ability doesnâ€™t trigger. If you donâ€™t have enough as the ability resolves, the ability has no effect.
If you activate Curious Homunculusâ€™s mana ability in response to its triggered ability, you can spend that mana to cast an instant spell during your upkeep after it has transformed into Voracious Reader. If you donâ€™t spend the mana, it will leave your mana pool before your draw step begins; you canâ€™t use it to cast the card you draw or to cast a sorcery.
For more information on double-faced cards, see the Shadows over Innistrad mechanics article (http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics).