As you wend your way through the Candy Crush Friends Saga path, you might find a mystery chest placed on a level number button.
But wait, the chest is locked!
Dachs the Donut Dog has the key, but is floating away further along the map. You'll need to collect the chest and unite it with the key to unlock it and collect the contents. But make a note of the time available to reach Dachs before he's out of reach as you'll need to get to him before the timer expires.
Once unlocked, inside you'll find fragment items which you can add to your collection and boost up the totals you need to finish each set.
If you miss the key - don't worry. You'll be able to hold onto the chest and when you see Dachs again, you'll get another chance to reach him in the time shown in the game.