At first I was hesitant to introduce this to my 6 year old daughter, thinking she would get frustrated ave give up on it quickly, but just the opposite happened! The progression of the levels is well designed so she barely even notices the increase in difficulty and wasn't disheartened at all when the monster pictures turned into numbers and variables. She absolutely devoured the app and beat the game, bonus levels and all, with 3 stars on all levels, in 2 days. She's looking forward to getting her next app as a reward. (Any suggestions on which one it should be? ) I'd usually balk at the cost, but I think the app is well worth the price. Thank you!