Heat always flows from hot to cold — but it can travel by several different mechanisms. Each panel below animates one of them so you can see how the energy actually moves.
Conduction, convection and radiation are the three fundamental physical mechanisms. Evaporation — energy carried off as a liquid's fastest molecules escape — is commonly counted as a fourth, especially for how bodies and buildings shed heat.
Through a solid, atom to atom. Heat the end of a metal bar and its atoms vibrate harder, jostling their neighbours and passing energy along the lattice — without the material itself moving.
Carried by a flowing fluid. Heated from below, warm fluid expands, becomes buoyant and rises; cooler fluid sinks to take its place, setting up circulating convection currents.
As electromagnetic waves. Every warm object emits infrared radiation that crosses empty space at the speed of light — no medium required. It's how the Sun's heat reaches the Earth.
The fastest molecules escape. At a liquid's surface the most energetic molecules break free as vapour, carrying their energy away. What's left behind is cooler — this is evaporative cooling.
Four independent animations, one per transfer mode. The conduction bar actually integrates the 1-D heat equation each frame; the others are faithful cartoons of the physical mechanism.
Conduction follows Fourier's law — heat current is proportional to the temperature gradient and the material's conductivity k:
Convection is usually modelled with Newton's law of cooling, where h bundles up the details of the flow:
Radiation follows the Stefan–Boltzmann law — every body above absolute zero emits, with power rising as the fourth power of temperature:
Evaporation removes the latent heat of vaporisation L with every kilogram of liquid that escapes as vapour:
Real objects almost always lose heat by several modes at once. A hot mug conducts into the table, drives convection in the surrounding air, radiates infrared, and steams off water — engineering a thermos means defeating all four.