The rocket engine, with its steady roar like that of a waterfall or a thunderstorm, is an impressive symbol of the new space age. Rocket engines have proved powerful ________(1) to shoot astronauts ________(2) the earth’s gravitational ________(3) and put them on the moon. We have now ________(4) space travelers.  

    Impressive and complex ________(5) it may appear, the rocket, which was ________(6) in China over 800 years ________(7), is a relatively simple device. Fuel that is ________(8) in the rocket engine changes ________(9) gas. The hot and rapidly expanding ________(10) must escape, but it can do so only ________(11) an opening that heads ________(12). As the gas is ________(13) with great force, it ________(14) the rocket in the ________(15) direction. Like the ________(16) of a gun when it is fired, it ________(17) the ________(18) of nature described by Sir Isaac Newton when he found that “________(19) every action, there is another equal and opposite ________(20).”