I like the gradual introduction of new obstacles and mechanics - The use of the transformer for reflecting the rock back was clever.  With the number of mechanics and obstacles, I think 10 levels was the sweet spot for this game. Tracking the total number of shots fired throughout the whole game when combined with the game's short length was a good idea, as that encourages retrying again for improving your score.

The graphics were very clean and simple, and the sound effects used adds to that charm.

The minor improvements that I would suggest is:

• to add some text on level 2 that explains that only the arrow keys move the slingshot left and right 
• to change the movement pattern of the sparks in level 9. The sparks in previous levels have a pattern that can be fully observed within a short timeframe. The sparks in level 9 initially move in one direction for a while, which gives the impression that they only flow in one direction. However, the sparks then reverse direction after a period of time, which can be surprising to first time players and can cause a miss if they fıre after the switch occurs. 

Overall, a short and fun game.

I like the idea of conserving your one rock to win the level! It adds strategy to the game and was fun to play, even though I'm a fairly impatient person. :')

Poor birds though, lol.