Archimedes’ Principle is a law of physics defined by Archimedes, stating that “Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.” This means that any object, at least partially submerged in a fluid1 (ex. water) will be forced upwards (buoyed) by a force that is equal to the weight of the fluid that the object displaces. For example, if a rock suspended by a string with a weight of 10 newtons, is lowered into a tub of water, and the amount of water it displaces weighs 3 newtons, then the buoyant force acting upon it is 3 newtons (equal to the weight of the water it displaced). This also means that the force the rock is exerting on the string is equal to its original weight (10 newtons) minus the buoyancy force (3 newtons): 10 - 3 = 7 newtons. This shows that buoyancy reduces how much objects appear to weigh.
Archimedes’ Principle can be applied in many ways to prove various things. It shows why a steel ship can float (the water displaced by its hull is less than or equal to its hull’s weight), why wood floats in water, and why a helium balloon drifts forward in a car moving forward even though all the air is moving backwards.
1 A fluid is a substance that continually deforms under an applied shear stress. The colloquial term implies a liquid, however in physics liquids, gases as well as plasmas and some plastic solids are considered fluids.