Can you react to Kaisa's Evolutionary trigger with Stupefy, then play that same Stupefy from trash using her ability?

Ruling: No, you cannot use the same Stupefy both as a reaction to the trigger and as the spell played by Kaisa's ability. The trigger targets a spell in the trash when it goes on the stack, before you have priority to react.

Sequence:

• Kaisa's trigger goes on the stack and targets a spell in the trash
• Players can now react to the trigger
• If you want to use the same Stupefy for both purposes, you would need to play it before Kaisa conquers

Nuances:

• You could double-use a single Stupefy if you play it before Kaisa conquers (so it's in the trash when her trigger resolves)