Have you ever wondered why hot air balloon floats in the air? Even though the mass of the balloon is big?
Archimedes (287-212 B.C.) was a greek scientist who first discovered that
"an object submerged in a liquid is acted on by an upward buoyant force (or upthrust)."
The buoyant force is due to the surrounding liquid which causes the object to weigh less in the liquid. Archimedes realised that submerged objects always displace liquid upwards, (when you put an ice to a glass of water, the water level rise). Later he did show that the upthrust is equal to the weight of water displaced.
Archimedes' principle states that an object, whether completely or partially immersed in a fluid, is acted on by a buoyant force, which is equal to the weight of the fluid displaced.
