Here's how it works:

1. Reaction: The rocket's fuel (often a mixture of water and air) is pressurized and heated, causing it to rapidly expand and become a hot gas.

2. Action: This expanding gas exerts a strong downward force on the bottom of the bottle.

3. Reaction: According to Newton's Third Law of Motion, for every action, there is an equal and opposite reaction. Therefore, the bottle pushes back on the expanding gas, pushing it out of the nozzle.

4. Result: This upward push of the gas out of the nozzle creates the thrust that propels the bottle rocket upward.

So, the action force is the downward force of the expanding gas, and the reaction force is the upward thrust that launches the rocket.