Pop bottles normally find their way into the trash or recycling but if you have some time you can make some interesting rockets with them. The 2 stage water Rocket above was launched to a height of 864 feet.

 
This one quick to build the rocket portion, most of the bottle work is for looks and a bit of stable flight purpose. The neck of the bottle is simply friction fit to the multiple wraps of duct tape on the pipe that increased the diameter to provide a snug fit. The end of the tube has a cork stuffed in the end with a bottle cap and filler nozzle glued in place. This adds some safety since the cork can easily pop out if over pressured. An alternative would have been to glue an end cap to the pipe and drill a hole to mount the filler nozzle.
 

 

 
This Oobleck filled water rocket didn’t behave the way I expected. Oobleck is liquid and can be poured but when a hard force is applied it firms up. The Oobleck reacts to the high pressure and delays the launch until it is slowly forced out of the nozzle.

 
This alcohol powered rocket is great since it can be fired with the push of a button. A simple BBQ lighter is used as the spark source, a bit of rubbing alcohol fumes is used to power it but this design should be great with other things such as propane and hair spray.