You can save 4 teensies per level. Teensies are the blue "animals". Depending on the amount of lums you collect in each level, you will save teensies.
- For just completing a level you save 1 teensie
- For 25 lums in a level you save 1 more Teensie (2 total)
- For 50 lums in a level you save 1 more Teensie (3 total)
- For 75 lums in a level you save 1 more Teensie (4 total)
(For 100 lums in a level you will unlock the advanced level, in which you can save another 4 teensies.)
You cannot replay levels to save the teensies again. You will need to play at least 3 levels for this achievement to unlock.