Solving puzzles ranks your performance out of four and gives you a corresponding number of stars. Collecting a certain number of stars unlocks more features:

Effect - Code

30 Puzzles - 120 Stars
30 Puzzles - 100 Stars
4 stars - Complete a Practice or Easy puzzle in 5 minutes or less
5 stars - Complete a Normal puzzle in 10 minutes or less
50 puzzles - 20 stars
6 stars - Complete a Hard puzzle in 15 minutes or less
Bronze Rank - Test Collect 60 stars
Gold Rank - Test Collect 220 stars and complete Bronze and Silver Rank Tests
Hard 101-110 - Collect 240 stars
Hard 111-120 - Collect 260 stars
Hard 91-100 - Collect 200 stars
Normal 141-160, Hard 81-90 - Collect 160 stars
Normal 161-190 - Collect 180 stars
Platinum Rank Test - Collect 300 stars and complete Bronze, Silver and Gold Rank Tests
Silver Rank Test - Collect 140 stars and complete Bronze Rank Test