I REALLY love the game idea of this. It has massive potential. Originally i didn't get the main idea but after realizing they swap places every time we shoot through them, it was like magic. Awesome!
It is pointed out in other comments that level 19 is hard and i agree too, i got stuck on it :P
I feel like i pushed more than grabbed it. Like it doesn't seem like the hassle to go to the 'E' key when you can just push it to the button but its a nice option.
Amazing game design, great job!