Good concept, but so incredibly frustrating to play. I died quite a few times just because my mouse was off center when I clicked 'go' and they instantly died.
It could use music and a way to reset, as has been mentioned before. It would also be nice to see some physics on the smoke when you fly through a trail you've already created.
Also more smoke colors (only white/yellow/red now?) and the ability to clear the trails would be good.
Weird bug... I tried to quit back to the menu and it just repeatedly said 'clear' and moved me to the next level...
This also occurred when I pressed the 'A' key in the level... I automatically got pushed to the next one.
The arrows fly by really fast and it's hard to enjoy the action while you're frantically watching for and mashing arrow keys. It's a good game, but maybe a longer song (or an adventure of several songs) and more focus on the knight's actions would be interesting.
I REALLY love the concept here. It's got some bugs that others have mentioned ("muscles" breaking and the 'sticky' not working ideally). Also, it'd be nice to have some different shapes, just for fun. But it's a really cool game concept.