When do you pick a unit target for a spell like Gust - when you play it or when it resolves?

Ruling: You pick the unit when you place the spell on the chain, not when it resolves.

Sequence:

• Choose the target when placing the spell on the chain
• Target legality is checked at that time (must be legal when chosen)
• Target legality is checked again when the spell resolves to ensure the target is still legal

Nuances:

• You cannot choose an illegal target initially with the intention of making it legal later
• If your opponent reacts to raise their units' might after you've chosen the target, the target has already been selected