Short, simple, but bloody efficient!
Those kind of visual effects are exactly what I have in mind with "merge", and also exactly what I feel absolutely unable to create (... yet). But you pulled it off apparently! The magic is here!
And it's used as a clever mechanic! It's easy to understand, fast, rewarding, and fun. I really liked it.
As basic as this game is, I'm impressed at how consistent it manages to be. Great work!