Epic Experiment

If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast the card.

You cast the cards one at a time, choosing modes, targets, and so on. The last card you cast will be the first one to resolve.

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.