Something that changes zones becomes a new object in that zone and will therefore be a new source. For example, if you cast Flametongue Kavu, deal damage with its enters-the-battlefield ability, exile and return it to the battlefield with Momentary Blink, deal damage again, then cast Momentary Blink with flashback on Flametongue Kavu to repeat the process one more time, you've fulfilled Case of the Burning Masks's "to solve" trigger condition. 