If one of Explosion's two targets becomes illegal, the other is affected as appropriate.

The copy will have the same targets as the spell it's copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can't choose a new legal target, then it remains unchanged (even if the current target is illegal).

If the spell that's copied is modal (that is, it says "Choose one —" or the like), the copy will have the same mode or modes. You can't choose different ones.

If the spell that's copied has an X whose value was determined as it was cast (like Banefire does), the copy will have the same value of X.

If the spell has damage divided as it was cast, the division can't be changed (although the targets receiving that damage still can). The same is true of spells that distribute counters.

The controller of a copy can't choose to pay any alternative or additional costs for the copy. However, effects based on any alternative or additional costs that were paid for the original spell are copied as though those same costs were paid for the copy.

Any choices made as a spell resolves won't have been made yet once it's copied. Any such choices will be made separately as the copy resolves.

If you copy a spell, you control the copy. It will resolve before the original spell does.

Expansion can copy any instant or sorcery spell with an appropriate mana value, not just one with targets.

If a spell has {X} in its mana cost, include the value chosen for that X when determining the mana value of that spell.

The copy is created on the stack, so it's not "cast." Abilities that trigger when a player casts a spell won't trigger.

Split cards have two card faces on a single card. The characteristics of the half you didn't cast are ignored while the spell is on the stack.

Each split card is a single card. For example, if you discard a split card, you've discarded one card, not two. If an effect counts the number of sorcery cards in your graveyard, Assault // Battery counts once, not twice.

Each split card has two names. If an effect instructs you to choose a card name, you may choose one of those names, but not both.

A split card's characteristics are a combination of its two halves while it is not on the stack. For example, Assault // Battery has a mana value of 5 while it is in your library. If an effect allows you to search your library for a card with mana value 4 or less, you can't find Assault // Battery.

If an effect allows you to cast a spell with certain characteristics, consider only the characteristics of the half you're casting. For example, if an effect allows you to cast a sorcery spell with mana value 2 or less from among cards in your graveyard, you could cast Assault this way, but not Battery.

If you copy a spell that's half of a split card, the copy copies that same half. For example, if you copy Assault, the copy is also Assault, not Battery.

To cast a split card, choose one of its halves to cast. There's no way to cast both halves of this split card.