Buoyancy

 Buoyancy is the "tendency of a body to float or to rise when submerged in a fluid," (Merriam-Webster). This phenomenon is the result of the increasing pressure of fluids at increasing depths. According to Pascal's Principle, this pressure applies in all directions; therefore, the pressure on the bottom of glass balls pictured is larger than that applied by air on the top of the balls, causing a net upward force on them.