This is a very simple explanation that I have given to my Grade 6 students. Bernoulli's Theory, which originally was applied to his research in fluids & medicine (particularly blood pressure), helps explain how planes fly. He discovered the principal that the pressure of a fluid decreases when its speed or velocity increase, this prinicpal was also later discovered to apply to air pressure as well. This helps us to understand that when a plane is speeding up for take off, the faster it goes the less air pressure is exerted on it and therefore the plane is able to 'lift-off' the ground and fly. A plane that does not obtain sufficient speed on the ground, for its size & weight, may not be able to take off successfully.