Simple, but very elegant in its execution. I agree with the other commenters that this already feels like a solid mobile puzzle game, and that having the option to replay previous levels would be a much-needed improvement.
What I really like is how there are multiple ways to solve each puzzle once the player has two shapes to choose from. You could probably create massive custom grid designs and let people figure out for themselves if they're solvable with the given five shapes (maybe being a daily challenge of sorts, if the game does get officially published).
I also like how the five shapes sort of spell the game's title (even though the "i" shape is more L-shaped). It didn't have to be done, but it just makes the game all the more charming to play through.