JACK has the correct answer.
I'd just like to add that the reason why I scratched all these holes is that I didn't spray enough water onto the ceiling at one go, so the bond between the popcorn and the ceiling was too tight for me to use a knife to separate them. I subsequently sprayed more water (little by little) and try to scratch. It only made things worse. So you see some of the ceiling is totally wet and some lost drywall paper.
What I should've done is to use a pump sprayer (cost $10) to spray enough water to the ceiling and then scratch.
After learning my mistake, the rest of the ceiling got scraped off very smoothly.