The chain
When you play a spell it does not just happen. It goes on the chain, and the other player gets a say before it resolves. Six worked chains, including the one where “play” means two different things.
Empty. Nothing is waiting to resolve.
The chain resolves top down — last played, first to happen. Triggers go on it too, not just spells. Every item is finalized (choices made, costs paid, including additional costs) before anything resolves, and cards care about one or the other: “when you play a spell” waits for it to resolve, while “if you’ve played a spell” is satisfied the moment it finalizes — even if it gets countered. One thing never waits on the chain at all: a unit or gear finalizes straight onto the board, with no window to answer the card itself. What you can answer is the trigger it arrives with, which goes on the chain afterwards like everything else.