It's actually something I like about the game - while playing through my first time, I was constantly thinking either "I wonder if anyone else thought to do this?" and "I bet everyone else figured this out instantly ;_;"
ill be here 4 u eco jus lyk the mop on the commercial babby-wechina23 fighting games suck-bluerain
There is a part that shows you that you can create extra blocks. However, I, like you, didn't get the hint from that part and got stuck on some of the puzzles that require you to do that (resource management, too many lasers)
"just be glad there's nobody named paperlikeabook to come and post after you. Then you'd be in trouble mister." - arleas http://imgur.com/QgO8q
There is one puzzle that shows this off before you need to do it yourself. In the puzzle, there's a bunch of lasers in a square like this
L-L |xx| L-L
L=laser source, | and - are laser lines. You have to block them all to trigger a door. However, you can't access the area the lasers are in directly. Instead, when you fill a slot around the entire thing with green blocks, they fill in the center and hit all the lasers. If you just filled the slot without paying attention to what was going on you might not notice them filling in the center though.
Endure. In enduring, grow strong. (Planescape: Torment)
The room that teaches you is called "Closing the Gap". There is a sign afterwards that reads: "Solving a problem may require using abilities that we didn't realize we had." That hints towards the fact the most people wouldn't realize that they can fill in blocks by drawing a perimeter with the green gun.