How does a hot-air balloon work (in theory)? In a word: buoyancy . Hot-air balloons float in the sky for pretty much the same reason that boats float on the sea. A boat floats because it's supported by the water beneath it: the weight of the boat (pulling downward) is exactly counterbalanced by the pressure of the water beneath it (pushing upward). A boat doesn't float perfectly on the water surface but sinks partly into the water according to how heavy it is. The bigger the boat, the bigger the area of water beneath it, the greater the force of the water pressure pushing upward on it, and the more weight it can carry. Here's another way to look at it: generally speaking, an object will float if it's less dense than water (in other words, lighter than an equal volume of water) and sink if it's more dense (heavier than an equal volume of water). Imagine a block of lead the size of your arm dropped into a bathtub filled with water. An "armful of lea...