These links describe how bottle rockets work.

http://www.bbc.co.uk/bang/handson/waterbottlerockets.shtml
https://www.grc.nasa.gov/WWW/K-12/rocket/rktbot.html

The following link is an extension of the NASA one above. It would be really good for you to visit this site. It is exactly about what we are doing. The difference is we have a payload and my launcher is a little more accurate and complex but that’s all.

https://www.grc.nasa.gov/WWW/K-12/rocket/BottleRocket/about.htm

You need to be careful when you are exploring bottle rockets. They are a projectile and as such, dangerous. When I made my launcher I was conscious I would have others around me who needed to be safe. I enclosed the sides so if a rocket failed and shrapnel would not go in a direction it could do any harm. I am always conscious to check wind direction before launch and ensure I am up wind from my landing site. 

https://www.grc.nasa.gov/WWW/K-12/rocket/BottleRocket/safety.htm

An interesting description of the work of Sir Isaac Newton. A bit technical but this is for interest only if you would like to know a little more about his laws of motion. When a theory has been tested and tested over and over with the same result then it becomes a law and until it is disprove it remains as such.

https://www.grc.nasa.gov/WWW/K-12/airplane/newton.html