This simulation portrays a vertical cross-section of a hypothetical fluid medium. The circular area in the center represents a region of the fluid that is being heated by a heat source.  The red region at the center indicates the hottest fluid, while blue indicates cooler fluid farther from the heat source. As the heated fluid expands, it becomes less dense, more buoyant, and rises, represented by bubbles.  Smaller, bluer bubbles represent higher density whereas larger, redder bubbles represent lower density.  Contrary to what one might expect, lower density actually translates into larger bubbles because as the fluid heats, it expands. As the fluid rises out of the heated region, it cools off, becomes more dense, and sinks.