Hot Air Balloon

Hot air balloons, those big floating ovals of vividly coloured nylon, rise up by convection.
A small propane flame at the base of the balloon heats the air near the opening.
As the air warms, it expands; its molecules spread out.
The hot air in the balloon is less dense—weighs less—than the surrounding cool air. So it rises above the cooler air like a bubble floating up through water.
Indeed, the force lifting the balloon can be determined the same way as the buoyant force pushing an object up in liquid: it's equal to the weight of the volume of cool air the balloon displaces.
The biggest balloons have the strongest lift. Of course, the balloonist must also have a way to return to Earth.
The pilot does this by pulling a cord that opens a valve at the top of the balloon, letting hot air escape.
The volume in the balloon decreases, and the balloon drops.