A milk carton filled with water is hanging from a string. what will happen if you punch two holes in opposite sides of the carton at the opposite corners?

When you punch holes in opposite corners of a milk carton filled with water which is hanging from a string, water will start to flow outside with a certain force. There will be a reaction equal and opposite to this force that will make the milk carton start to spin.

This is a typical example of water turbine that is used to demonstrate the third Newton's law.