DemonstrateDemonstrate is a triggered ability. “Demonstrate” means “When you cast this spell, you may copy it and you may choose new targets for the copy. If you copy the spell, choose an opponent. That player copies the spell and may choose new targets for that copy.” (CR 702.144) (When you
castTo cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. A player may cast a spell if they have priority. See rule 601, “Casting Spells.”
To cast a card is to cast it as a spell. (CR 701.5) this spell, you may copy it. If you do, choose an opponent to also copy it. Players may choose new targets for their copies.)
DestroyThe only ways a permanent can be destroyed are as a result of an effect that uses the word “destroy” or as a result of the state-based actions that check for lethal damage (see rule 704.5g) or damage from a source with deathtouch (see rule 704.5h). If a permanent is put into its owner’s graveyard for any other reason, it hasn’t been “destroyed.”
A regeneration effect replaces a destruction event. See rule 701.19, “Regenerate.” (CR 701.8) target artifact or creature you don't control. If that permanent is destroyed this way, its controller exiles cards from the top of their library until they
exileTo exile an object, move it to the exile zone from wherever it is. See rule 406, “Exile.” (CR 701.13) a nonland card, then they may
castTo cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. A player may cast a spell if they have priority. See rule 601, “Casting Spells.”
To cast a card is to cast it as a spell. (CR 701.5) that card without paying its mana cost.