bernoulli's principle is a rule in fluid dynamics that says when the speed of a fluid (like air or water) increases, its pressure decreases. it's based on the idea that energy in a moving fluid is conserved —if the fluid flows faster, the energy shifts from pressure into motion. you can see this in everyday life: airplanes use it to create lift (air moves faster over the curved top of the wing, lowering pressure there, while slower air under the wing keeps pressure higher, pushing the plane up).