We've all thought it, in fact most adults consider this question every time they board a plane. It's not so much a problem when you're younger, as a child flying is simply the exciting prelude to your summer holiday, you're not of an age where physics and aerodynamics trouble your mind so much, you just stare out the window and marvel at the what the world looks like from so high up. But as soon as you reach early adulthood the phrase - a tin can with wings - keeps popping into your mind.

But once you know about the science behind air travel you realise that it's good old reliable physics that are pretty hard to get wrong. There are four main forces that keep a plane in the air, these are lift, weight, thrust and drag. Lift pushes the plane upwards, this is dependent on the wings and their shape because it is the way the air moves around the wings that lifts the aircraft up. Both the upside and underside of a plane's wing are vital in redirecting the flow of air and keeping it up. When discussing aerodynamics air is often referred to as fluid. Lift occurs when a flow of fluid i.e. the air, is deflected by a solid object. This creates lift in one direction while flow occurs in the opposite path. Any solid surface can make the flow of fluid to turn, and this guarantees that the aircraft remains in the sky and doesn't plummet to the ground. For example, both the upper and lower portions of an airplane's wing surface are important in deflecting the flow of fluid.

The force of weight pulls the plane down towards Earth and to this end aircraft are engineered in order to keep the weight spread evenly across the whole of the plane's body, from the front to the back, and this maintains balance in the plane.

Of course an airplane can't just stay up in the air it also has to move forward so that holiday makers can actually have their holiday. The force behind this is thrust, and powerful turbo engines provide this thrust either in the form of a propeller turning or a jet engine gobbling up fuel, it doesn't really matter as long as air continues to glide over the plane's wings. But being propelled forward is only any good if you can stop, this is where the fourth force comes into play - drag. All commercial aircraft are designed to allow air to pass around them with less drag, this is their aerodynamic shape.

Planes fly when all four forces work together, but of course this is not the only ingredients of a successful flight, the other important part is of course the pilot. But out of the four forces the crucial one is probably lift, which works opposite weight.