Can you play a Hidden card in response to a Temporary keyword trigger (specifically, can you flip Teemo in response to a Sprite with Temporary dying)?

Ruling: Yes, you can play a Hidden card in response to a Temporary keyword trigger because playing a Hidden card is at Reaction speed, and Reactions can be played whenever there is a chain.

Sequence:

• At start of turn, Temporary starts a chain as a pending item
• This opens up an opportunity to play a Reaction
• Reveal the Hidden card as a Reaction
• Hidden card resolves first, then Temporary resolves

Nuances:

• The card must have been Hidden on a prior turn, not the current turn
• For abilities that require targets (like blue Teemo's damage effect), the ability won't activate if there are no valid targets on the battlefield, even if other parts of the ability text are separated by periods
• Purple Teemo will still get the +3 stat bonus when played from Hidden in this scenario