Why is a rocket designed in multi-stages?

Answer 1

To lift a space vehicle into orbit requires massive thrust. The best way to achieve break-out of earths gravity is a lift-off vehicle with a number of engines and a huge fuel tank. As soon as stage 1 has burned its fuel and done its job it is jettisoned, you don't need it, into the ocean it goes. Stage2 is fired to take you high into space and onto a course for your destination. Once that is achieved stage 2 is also jettisoned. Your remaining rocket is hardly needed,there is nothing to slow your progress in the vacuum of space. It will only be fired to alter course, manouvre, or bring you home. You just do not have the power to lift that vehicle off the ground, into orbit, and onto its destination in its entirety, so we do it in sections