Heat flows from "hot to cold" substances.
Explanation:
"Heat flows from hot to cold" is the statement of "Second Law of Thermodynamics".
According to this law, the heat flow is due to the collision of hot atoms collides with cold atoms transferring kinetic energy. When the heat flows from "hot to cold", external energy is not required for the flow, but when it flows from cold to hot, it needs external energy in which the spontaneity of heat flow is less.