Because the .jars are Compiled versions of your code. They are written a way that only java can read, and then convert that into something the computer can read. It's not meant for human reading. Yes, you can open .jars in greenfoot, but you can't edit them, because greenfoot doesn't know what to do with the illegible code. If you want to remake the scenario, you'll have to start from scratch.

I don't know if I'm right, but I'm guessing some of the lag may be coming from the sound. I'm getting really glitchy sound, don't know if that's what you wanted, or if my headphones are finally completely broke, but it's clear sometimes, and really weird sounding at others. -Happy Birthday to me!

Pretty good for one of your first scenarios. (By the way, really like you're sprites) I recommend you make it that the bullets come from the gun, instead of where the mouse is.

This is cool! Took me a while to figure out I had to put a URL in... Guess I wasn't paying good enough attention. -Happy Birthday to me!